Quantum dots of group IV semiconductor materials

ABSTRACT

The invention relates to a quantum dot. The quantum dot comprises a core including a semiconductor material Y selected from the group consisting of Si and Ge. The quantum dot also comprises a shell surrounding the core. The quantum dot is substantially defect free such that the quantum dot exhibits photoluminescence with a quantum efficiency that is greater than 10 percent.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application Ser.No. 60/309,898, filed on Aug. 2, 2001, U.S. Provisional Application Ser.No. 60/309,905, filed on Aug. 2, 2001, U.S. Provisional Application Ser.No. 60/309,979, filed on Aug. 2, 2001, U.S. Provisional Application Ser.No. 60/310,090, filed on Aug. 2, 2001, and U.S. Provisional ApplicationSer. No. 60/310,095, filed on Aug. 2, 2001, the disclosures of which areincorporated herein by reference in their entirety.

FIELD OF THE INVENTION

This invention relates generally to quantum dots. More particularly,this invention relates to quantum dots of Group IV semiconductormaterials.

BACKGROUND OF THE INVENTION

Over the past several years, there has been an increasing interest inexploiting the extraordinary properties associated with quantum dots. Asa result of quantum confinement effects, properties of quantum dots candiffer from corresponding bulk values. These quantum confinement effectsarise from confinement of electrons and holes along three dimensions.For instance, quantum confinement effects can lead to an increase inenergy gap as the size of the quantum dots is decreased. Consequently,as the size of the quantum dots is decreased, light emitted by thequantum dots is shifted towards higher energies or shorter wavelengths.By controlling the size of the quantum dots as well as the materialforming the quantum dots, properties of the quantum dots can be tunedfor a specific application.

Previous attempts at forming quantum dots have largely focused onquantum dots of direct band gap semiconductor materials, such as GroupII-VI semiconductor materials. In contrast to such direct band gapsemiconductor materials, Group IV semiconductor materials such as Si andGe have energy gaps, chemical properties, and other properties thatrender them more desirable for a variety of applications. However,previous attempts at forming quantum dots of Si or Ge have generallysuffered from a number of shortcomings. In particular, formation ofquantum dots of Si or Ge sometimes involved extreme conditions oftemperature and pressure while suffering from low yields and lack ofreproducibility. And, quantum dots that were produced were generallyincapable of exhibiting adequate levels of photoluminescence that can betuned over a broad spectral range. Also, previous attempts havegenerally been unsuccessful in producing quantum dots of Si or Ge thatare sufficiently stable under ambient conditions or that can be madesufficiently soluble in a variety of matrix materials.

It is against this background that a need arose to develop the quantumdots and methods for forming quantum dots described herein.

SUMMARY OF THE INVENTION

In one innovative aspect, the present invention relates to a quantumdot. In one embodiment, the quantum dot comprises a core including asemiconductor material Y selected from the group consisting of Si andGe. The quantum dot also comprises a shell surrounding the core. Thequantum dot is substantially defect free such that the quantum dotexhibits photoluminescence with a quantum efficiency that is greaterthan 10 percent.

In another embodiment, the quantum dot comprises a core including asemiconductor material Y selected from the group consisting of Si andGe. The quantum dot also comprises a ligand layer surrounding the core.The ligand layer includes a plurality of surface ligands. The quantumdot exhibits photoluminescence with a quantum efficiency that is greaterthan 10 percent.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the nature and objects of the invention,reference should be made to the following detailed description taken inconjunction with the accompanying drawings, in which:

FIGS. 1(a), 1(b), 1(c), and 1(d) illustrate quantum dots according tosome embodiments of the invention.

FIG. 2 illustrates the energy gap of quantum dots fabricated fromsilicon plotted as a function of the size of the quantum dots, accordingto an embodiment of the invention.

FIG. 3 illustrates photoluminescence (PL) spectra from six samples withdifferent sizes of silicon quantum dots, according to an embodiment ofthe invention.

FIG. 4(a) illustrates the energy gap of quantum dots fabricated fromgermanium plotted as a function of the size of the quantum dots,according to an embodiment of the invention.

FIG. 4(b) illustrates size-selective photoluminescence (PL) spectra fordifferent sizes of germanium quantum dots, according to an embodiment ofthe invention.

FIG. 5(a) illustrates concentration dependence of the linear index ofrefraction of engineered nonlinear nanocomposite materials doped withsilicon and germanium quantum dots, according to an embodiment of theinvention.

FIG. 5(b) illustrates concentration dependence of the opticalnonlinearity of engineered nonlinear nanocomposite materials doped withsilicon and germanium quantum dots, according to an embodiment of theinvention.

FIGS. 6(a), 6(b), 6(c), 6(d), 6(e), and 6(f) illustrate nonlineardirectional couplers comprising engineered nonlinear nanocompositematerials, according to some embodiments of the invention.

FIGS. 7(a), 7(b), 7(c), 7(d), 7(e), and 7(f) illustrate an embodiment ofa nonlinear Mach-Zehnder (MZ) interferometer comprising an engineerednonlinear nanocomposite material.

FIGS. 8(a), 8(b), 8(c), and 8(d) illustrate an alternative embodiment ofa nonlinear MZ interferometer comprising an engineered nonlinearnanocomposite material.

FIG. 9 illustrates a figure-of-merit (FOM) for all-optical switchingwith an engineered nonlinear nanocomposite material as a function ofquantum dot size, according to an embodiment of the invention.

FIGS. 10(a) and 10(b) illustrate photoluminescence spectra of siliconquantum dots made in accordance with an embodiment of the invention.

FIGS. 11(a) and 11(b) illustrate photoluminescence spectra of germaniumquantum dots made in accordance with an embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

Definitions

The following definitions may apply to some of the elements describedwith regard to some embodiments of the invention. These definitions maylikewise be expanded upon herein.

As used in this specification and the appended claims, the singularforms “a”, “an”, and “the” include plural references unless the contentclearly dictates otherwise. Thus, for example, reference to “a quantumdot” includes a mixture of two or more such quantum dots and may includea population of such quantum dots.

“Optional” or “optionally” means that the subsequently described eventor circumstance may or may pot occur and that the description includesinstances where the event or circumstance occurs and instances in whichit does not. For example, the phrase “optionally surrounded with ashell” means that the shell may or may not be present and that thedescription includes both the presence and absence of such a shell.

Embodiments of the invention relate to a class of novel materialscomprising quantum dots. As used herein, the terms “quantum dot”, “dot”,and “nanocrystal” are synonymous and refer to any particle with sizedependent properties (e.g., chemical, optical, and electricalproperties) along three orthogonal dimensions. A quantum dot can bedifferentiated from a quantum wire and a quantum well, which havesize-dependent properties along at most one dimension and twodimensions, respectively.

It will be appreciated by one of ordinary skill in the art that quantumdots can exist in a variety of shapes, including but not limited tospheroids, rods, disks, pyramids, cubes, and a plurality of othergeometric and non-geometric shapes. While these shapes can affect thephysical, optical, and electronic characteristics of quantum dots, thespecific shape does not bear on the qualification of a particle as aquantum dot.

For convenience, the size of quantum dots can be described in terms of a“diameter”. In the case of spherically shaped quantum dots, diameter isused as is commonly understood. For non-spherical quantum dots, the termdiameter, unless otherwise defined, refers to a radius of revolution(e.g., a smallest radius of revolution) in which the entirenon-spherical quantum dot would fit.

A quantum dot will typically comprise a “core” of one or more firstmaterials and can optionally be surrounded by a “shell” of a secondmaterial. A quantum dot core surrounded by a shell is referred to as a“core-shell” quantum dot.

The term “core” refers to the inner portion of the quantum dot. A corecan substantially include a single homogeneous monoatomic or polyatomicmaterial. A core can be crystalline, polycrystalline, or amorphous. Acore may be “defect” free or contain a range of defect densities. Inthis case, “defect” can refer to any crystal stacking error, vacancy,insertion, or impurity entity (e.g., a dopant) placed within thematerial forming the core. Impurities can be atomic or molecular.

While a core may herein be sometimes referred to as “crystalline”, itwill be understood by one of ordinary skill in the art that the surfaceof the core may be polycrystalline or amorphous and that thisnon-crystalline surface may extend a measurable depth within the core.The potentially non-crystalline nature of the “core-surface region” doesnot change what is described herein as a substantially crystalline core.The core-surface region optionally contains defects. The core-surfaceregion will preferably range in depth between one and five atomic-layersand may be substantially homogeneous, substantially inhomogeneous, orcontinuously varying as a function of position within the core-surfaceregion.

Quantum dots may optionally comprise a “shell” of a second material thatsurrounds the core. A shell can include a layer of material, eitherorganic or inorganic, that covers the surface of the core of a quantumdot. A shell may be crystalline, polycrystalline, or amorphous andoptionally comprises dopants or defects. The shell material ispreferably an inorganic semiconductor with a bandgap that is larger thanthat of the core material. In addition, preferred shell materials havegood conduction and valence band offsets with respect to the core suchthat the conduction band is desirably higher and the valence band isdesirably lower than those of the core. Alternatively, the shellmaterial may have a bandgap that is smaller than that of the corematerial, and/or the band offsets of the valence or conduction bands maybe lower or higher, respectively, than those of the core. The shellmaterial may be optionally selected to have an atomic spacing close tothat of the core material.

Shells may be “complete”, indicating that the shell substantiallycompletely surrounds the outer surface of the core (e.g., substantiallyall surface atoms of the core are covered with shell material).Alternatively, the shell may be “incomplete” such that the shellpartially surrounds the outer surface of the core (e.g., partialcoverage of the surface core atoms is achieved). In addition, it ispossible to create shells of a variety of thicknesses, which can bedefined in terms of the number of “monolayers” of shell material thatare bound to each core. A “monolayer” is a term known in the artreferring to a single complete coating of a shell material (with noadditional material added beyond complete coverage). For certainapplications, shells will preferably be of a thickness betweenapproximately 0 and 10 monolayers, where it is understood that thisrange includes non-integer numbers of monolayers. Non-integer numbers ofmonolayers can correspond to the state in which incomplete monolayersexist. Incomplete monolayers may be either homogeneous or inhomogeneous,forming islands or clumps of shell material on the surface of thequantum dot. Shells may be either uniform or nonuniform in thickness. Inthe case of a shell having nonuniform thickness, it is possible to havean “incomplete shell” that contains more than one monolayer of shellmaterial. For certain applications, shell thickness will preferablyrange between approximately 1 Å and 100 Å.

It will be understood by one of ordinary skill in the art that there istypically a region between the core and shell referred to herein as an“interface region”. The interface region may comprise an atomicallydiscrete transition between the material of the core and the material ofthe shell or may comprise an alloy of the materials of the core andshell. The interface region may be lattice-matched or unmatched and maybe crystalline or noncrystalline. The interface region may contain oneor more defects or be defect-free. The interface region may behomogeneous or inhomogeneous and may comprise chemical characteristicsthat are graded between the core and shell materials such that a gradualor continuous transition is made between the core and the shell.Alternatively, the transition can be discontinuous. The width of theinterface region can range from an atomically discrete transition to acontinuous graded alloy of core and shell materials that are purely corematerial in the center of the quantum dot and purely shell material atthe outer surface. Preferably, the interface region will be between oneand five atomic layers thick.

A shell may optionally comprise multiple layers of a plurality ofmaterials in an onion-like structure, such that each material acts as ashell for the next-most inner layer. Between each layer there isoptionally an interface region. The term “shell” is used herein todescribe shells formed from substantially one material as well as aplurality of materials that can, for example, be arranged as multi-layershells.

A quantum dot may optionally comprise a “ligand layer” comprising one ormore surface ligands (e.g., organic molecules) surrounding a core of thequantum dot. A quantum dot comprising a ligand layer may or may not alsocomprise a shell. As such, the surface ligands of the ligand layer maybind, either covalently or non-covalently, to either the core or theshell material or both (in the case of an incomplete shell). The ligandlayer may comprise a single type of surface ligand (e.g., a singlemolecular species) or a mixture of two or more types of surface ligands(e.g., two or more different molecular species). A surface ligand canhave an affinity for, or bind selectively to, the quantum dot core,shell, or both at least at one point on the surface ligand. The surfaceligand may optionally bind at multiple points along the surface ligand.The surface ligand may optionally contain one or more additional activegroups that do not interact specifically with the surface of the quantumdot. The surface ligand may be substantially hydrophilic, substantiallyhydrophobic, or substantially amphiphilic. Examples of the surfaceligand include but are not limited to an isolated organic molecule, apolymer (or a monomer for a polymerization reaction), an inorganiccomplex, and an extended crystalline structure.

It will be understood by one of ordinary skill in the art that whenreferring to a population of quantum dots as being of a particular“size”, what is meant is that the population is made up of adistribution of sizes around the stated “size”. Unless otherwise stated,the “size” used to describe a particular population of quantum dots willbe the mode of the size distribution (i.e., the peak size).

As used herein, the “size” of a quantum dot will refer to the diameterof a core of the quantum dot. If appropriate, a separate value will beused to describe the thickness of a shell surrounding the core. Forinstance, a 3 nm silicon quantum dot with a 1.5 nm SiO₂ shell is aquantum dot comprising a 3 nm diameter core of silicon surrounded by a1.5 nm thick layer of SiO₂, for a total diameter of 6 nm.

For certain applications, the thickness of the ligand layer is a singlemonolayer or less and can sometimes be substantially less than a singlemonolayer.

As used herein, the term “photoluminescence” refers to the emission oflight of a first wavelength (or range of wavelengths) by a substance(e.g., a quantum dot) that has been irradiated with light of a secondwavelength (or range of wavelengths). The first wavelength (or range ofwavelengths) and the second wavelength (or range of wavelengths) can bethe same or different.

As used herein, the term “quantum efficiency” refers to the ratio of thenumber of photons emitted by a substance (e.g., a quantum dot) to thenumber of photons absorbed by the substance.

As used herein, the term “monodisperse” refers to a population ofquantum dots wherein at least about 60% of the population, preferably75% to 90% of the population, or any integer or noninteger therebetween,falls within a specified particle size range. A population ofmonodispersed particles deviates less than 20% root-mean-square (rms) indiameter, more preferably less than 10% rms, and most preferably lessthan 5% rms.

“Optically pure” refers to a condition in which light passing through orpast a material is substantially unchanged in mode quality as a resultof inhomogeneities in the material or modulations at the interfacebetween materials. This does not include mode disruption resulting fromchanges in index of refraction of waveguides. For instance, a materialwith large aggregates of quantum dots capable of scattering light wouldnot be optically pure. The same material with aggregates of a size thatdo not significantly scatter light, however, would be optically pure. Itwill be apparent to one of ordinary skill in the art that what is meantabove by “substantially unchanged” will depend on the opticalrequirements of a particular application. To this end, “optically pure”refers to the level of optical purity required for the application inwhich the material is to be used.

“Optically homogeneous” is defined as being homogeneous across a lengthscale that is significant for optical waves, preferably greater than 250nm, more preferably greater than 4 μm, and most preferably greater than˜1000 μm.

A “waveguide structure” is a term of art and refers to an optical devicecapable of transmitting light from one location to another. A waveguidestructure can transmit light through the use of guiding by localizedeffective index differences. One example of this involves total internalreflection within a “waveguide core”, with an index of refraction n₁,surrounded by a “cladding”, with an index of refraction n₂, whereinn₁>n₂. Another example of a waveguide structure involves appropriatelymicro or nanostructured materials such as photonic bandgap materialswhere the guiding results from the periodic micro- or nano-structure ofthe materials.

“Cladding” is any material that surrounds the waveguide core in awaveguide structure such that n₁>n₂. In a typical waveguide structure,light propagates as a traveling wave within and along the length of the“waveguide core” and evanescently decays within the cladding with adecay constant related to the ratio of n₁ to n₂. Light trapped within,and traveling along, the length of a waveguide core is referred to asbeing “guided”.

The shape of a waveguide core or a cladding can typically be describedin terms of its “cross-section”. The cross-section is the shape createdby cutting the waveguide core or the cladding along the axesperpendicular to the longitudinal axis of the waveguide structure. Thelongitudinal axis is the axis in which guided light travels.

“Optical fibers” and “planar waveguides” are two common forms ofwaveguide structures known in the art. “Optical fiber”, as the term iscommonly used, typically refers to a structure comprising asubstantially cylindrical waveguide core surrounded by a substantiallycylindrical cladding and optionally comprising a flexible, protectiveouter-coating. Alternatively, or in conjunction, an optical fiber cancomprise a non-cylindrical waveguide core with a cross-section shaped asa trapezoid, a circle, an oval, a triangle, or another geometric andnongeometric shape.

“Planar waveguides” are waveguide structures fabricated on a substrateby a variety of methods. “Planar waveguides” typically comprise asubstantially rectangular waveguide core. Alternatively, or inconjunction, planar waveguides can comprise non-rectangular waveguidecores with cross-sections of trapezoids, circles, ovals, triangles, or aplurality of other geometric and nongeometric shapes. While the term“planar” suggests a flat structure, the term “planar waveguide”, as usedherein, also refers to structures comprising multiple flat layers.Optionally, one or more layers in a planar waveguide are not flat. Oneof skill in the art will appreciate that the key aspect of a “planarwaveguide” is that it is a waveguide structure fabricated on a“substrate”. Unless otherwise stated, the term “waveguide structure”will be used herein to describe a planar waveguide.

“Waveguide substrate” or “substrate” is used herein to describe thematerial on which a planar waveguide is located. It is common that aplanar waveguide is fabricated directly on the surface of the substrate.The substrate typically comprises a solid support such as, for example,a silicon wafer and optionally comprises an additional “buffer layer”that separates the waveguide structure from the solid support. Thebuffer layer optionally comprises a plurality of layers comprising oneor more materials or combination of materials. The buffer layer mayoptionally act, in part, as a cladding. Alternatively, the waveguidesubstrate may be a flexible substrate serving the same purpose.

“Single mode” waveguide structures are those waveguide structures(either planar or fiber optic) that typically support a single opticalmode (e.g., TEM00). Such waveguide structures are preferred according tosome embodiments of the invention. “Multi-mode” waveguide structures arethose waveguide structures that typically support multiple optical modessimultaneously.

“Waveguide diameter” is herein used to describe the diameter of asubstantially cylindrical waveguide core of an optical fiber. Waveguidediameter is also used to describe the diameter of a substantiallycylindrical core on a planar waveguide.

“Waveguide width” or “width” is used herein to describe thecross-sectional dimension of a substantially rectangular waveguide corethat is oriented parallel to the substrate surface. This is alsoreferred to as the “horizontal dimension” of the waveguide core.“Waveguide height” or “height” is used herein to describe thecross-sectional dimension of a substantially rectangular waveguide corethat is oriented perpendicular to the substrate surface. This is alsoreferred to as the “vertical dimension” of the waveguide core. Based onthe definitions of “width” and “height” described here, one of ordinaryskill in the art will understand the translation of these terms to othergeometrically or nongeometrically shaped waveguide cores. Unlessotherwise stated, the standard definitions of width and height used ingeometry will be used to describe geometric cross-sectional shapes.

“Core taper” refers to a region of the waveguide core in which thegeometry of the waveguide core is changed. This may comprise changingthe size and/or shape of the waveguide core in one or two dimensions. Acore taper, for example, may comprise a transition of a waveguide corewith a square cross-section of 15 μm×15 μm to a waveguide core with asquare cross-section of 7 μm×7 μm. A core taper may also, for example,comprise a transition from a waveguide core with a square cross-sectionof 15 μm×15 μm to a waveguide core with a circular cross-section of 10μm in diameter. Many other forms of core-tapers are possible and will beunderstood from the above definition.

A “core taper” is typically engineered to gradually change thecharacteristics of the waveguide structure over a defined distance,referred to as the “taper length”. Ideally, the taper length will belong enough so that the transition preserves the mode structure of anoptical signal through the taper. In particular, it is preferred, butnot required, that a single optical mode entering a taper remains asingle mode after exiting the taper. This retention of themode-structure is referred to as an “adiabatic transition”. While theterm “adiabatic transition” is commonly used, those of ordinary skill inthe art will recognize that it is typically not possible to have aperfectly adiabatic transition, and that this term can be used todescribe a transition in which the mode structure is substantiallyundisrupted.

A “cladding taper” is a novel embodiment disclosed herein that issimilar to a core taper; however, it refers to a change in width of thecladding around the waveguide core. Similar to a core taper, a claddingtaper can be used to change the size and/or shape of the cladding andcan be defined to have a taper length. The taper length can be such asto produce an adiabatic or nonadiabatic transition.

Both core and cladding tapers may optionally refer to the case in whichthe index of refraction of the materials in the core or cladding aregradually changed, or “graded” over the taper length. As used herein,the term “gradually” refers to changes that occur continuously or insmall steps over a given nonzero distance. Core and cladding tapers mayoptionally comprise changes to the index, size, and/or shape of the coreor cladding, respectively.

A “bend” is used herein to describe a portion of a planar waveguide inwhich the planar waveguide displays a degree of curvature in at leastone dimension. Typically, the cross-section of the waveguide issubstantially unchanged within the bend. Typically, bends will be smoothand continuous and can be described in terms of a radius of curvature atany given point within the bend. While bends can curve the planarwaveguide both parallel and perpendicular to the substrate (e.g.,horizontal or vertical bends, respectively), unless otherwise stated,the term “bend” will herein refer to horizontal bends. Optionally, bendscan also comprise tapers.

A “multimode interference device” or multimode interferometer (MMI)refers to an optical device in which the cross-section of the waveguidecore is substantially changed (typically increased) within a shortpropagation length, leading to a region of waveguide core in which morethan one mode (but typically fewer than 10 modes) may propagate. Theinteraction of these propagating multiple modes defines the functionperformed by the MMI. MMI devices include fixed ratiosplitters/combiners and wavelength multiplexers/demultiplexers.

As used herein, a “waveguide coupler”, “optical coupler”, and“directional coupler” are synonymous and refer to a waveguide structurein which light is evanescently coupled between two or more waveguidecores within a coupling region such that the intensity of the lightwithin each of the individual cores oscillates periodically as afunction of the length of the coupling region. A more detaileddescription of a waveguide coupler is disclosed below.

A “nonlinear waveguide coupler” is a waveguide coupler in which theregion between and/or around two or more coupled waveguide cores isfilled with a material (e.g., an “active material”) with an index ofrefraction that can be changed. By changing the index of refraction ofthe active material, the coupling characteristics of the nonlinearwaveguide coupler can be modified. Alternatively, the active materialmay be contained within one or more of the coupled waveguide cores(e.g., as a section of one of the waveguide cores).

A “Mach-Zehnder interferometer” or “MZ interferometer” (MZI) is awaveguide structure in which light from a waveguide core (e.g., an“input waveguide core”) is split into two or more separate waveguidecores (e.g., “waveguide arms” or “arms”). Light travels a defineddistance within the arms and is then recombined into a waveguide core(e.g., an “output waveguide core”). In a MZ interferometer, the historyof the optical signals in each arm affects the resulting signal in theoutput waveguide core. A more detailed description of a MZinterferometer is disclosed below.

A “nonlinear MZ interferometer” is a MZ interferometer in which one ormore of the waveguide arms comprise an active material. The activematerial may be in the core and/or cladding of the waveguide arm.Modifying the index of refraction of the active material modulates thesignal in the output waveguide core by changing the degree ofconstructive and/or destructive interference from the waveguide arms.

“Active material” refers to any material with nonlinear opticalproperties that can be used to manipulate light in accordance with someembodiments of the invention. While the term active material willtypically be used to refer to an engineered nonlinear nanocompositematerial as described herein, the term may also be used to describeother nonlinear materials known in the art.

“Active region” refers to the region of an optical device in which theindex of refraction of the active material is modulated in order tomanipulate light. In the case of an electro-optic modulator, the activeregion is that area of the device where a voltage is applied. In a χ⁽³⁾based device, the active region is that area to which a trigger-signalis applied. Note that while the active region can be the only region ofthe device in which an intentional change in optical properties occurs,it does not restrict the location of the active material, which mayextend beyond the active region. Regions containing active materialsoutside the active region are typically not modulated during normaloperation of the device. “Active length” describes the length of theactive region along the longitudinal axis of the device.

In the case of optical devices employing evanescent coupling of lightbetween two waveguide cores (e.g., a waveguide coupler), the“interaction region” or “coupling region” is the region of the opticaldevice in which the coupling occurs. As is typically understood in theart, all waveguides can couple at some theoretically non-zero level. Theinteraction region, however, is typically considered to be that regionof the optical device in which evanescent fields of the waveguidesoverlap to a significant extent. Here again, the interaction region doesnot restrict the extent of either the active region or the activematerial, which may be greater or lesser in extent than the interactionregion.

“Interaction length” describes the length of the interaction region.“Interaction width” is the spacing between two coupled waveguides withinthe interaction region. Unless otherwise stated, the interaction widthis assumed to be substantially constant across at least a portion of theinteraction length.

“Trigger pulse”, “trigger signal”, “control pulse”, “control signal”,“control beam”, and “activation light” are synonymous and refer to lightthat is used to create a transient change in the index of refraction inthe materials of some embodiments of the present invention. A triggerpulse can either be pulsed or CW.

“Data pulse”, data signal”, and “data beam” are synonymous and refer tolight used to transmit information through an optical device. A datapulse can optionally be a trigger pulse. A Data pulse can either bepulsed or CW.

“CW light” and “CW signal” are synonymous and refer to light that is notpulsed.

“Wavelength range-of-interest” refers to any range of wavelengths thatwill be used with a particular optical device. Typically, this willinclude both the trigger and data signals, where the ranges for thetrigger and data signals can be the same or different. For instance, ifa device is fabricated for use in the 1550 nm telecom range, the datawavelength range-of-interest may be defined as 1.5 μm to 1.6 μm, and thetrigger wavelength range-of-interest may be defined as 1.5 μm to 1.6 μm(or a different range). For devices in the 1300 nm range, the datawavelength range-of-interest may be defined as 1.25 μm-1.35 μm. Whilethese are preferred wavelength range-of-interests, it will be understoodthat the specific wavelength range-of-interest can be differentdepending on the specific application. The ability to tune the materialsof embodiments of the current invention implies that any wavelengthrange-of-interest may be used. In general, 300 nm to 4000 nm is apreferred wavelength range-of-interest, more preferably 300 nm to 2000nm, more preferably 750 nm to 2000 nm, more preferably 1260 nm to 1625nm, most preferably 1310±50 nm and 1580±50 nm.

Quantum Dots

Embodiments of the current invention, in part, exploit the extraordinaryproperties of quantum dots. Quantum dots have optical and electronicproperties that can be dependent (sometimes strongly dependent) on boththe size and the material forming the quantum dots.

In nature, it is the size range on the order of a few nanometers inwhich the quantum mechanical characteristics of atoms and moleculesoften begin to impact and even dominate the classical mechanics ofeveryday life. In this size range, a material's electronic and opticalproperties can change and become dependent on size. In addition, as thesize of a material gets smaller, and therefore more atomic-like, manycharacteristics change or are enhanced due to a redistribution ofoscillator strength and density of states. These effects are referred toas “quantum confinement” effects. For example, quantum confinementeffects can cause the energy gap of the quantum dot or the energy of thelight emitted from the quantum dot to increase as the size of thequantum dot decreases. These quantum confinement effects result in theability to finely tune many properties of quantum dots (e.g., opticaland electronic properties) by carefully controlling their size. Thiscontrol provides one critical aspect of some embodiments of the currentinvention.

A quantum dot will typically be in a size range between about 1 nm andabout 1000 nm in diameter or any integer or fraction of an integertherebetween. Preferably, the size will be between about 1 nm and about100 nm, more preferably between about 1 nm and about 50 nm or betweenabout 1 nm to about 20 nm (such as about 2, 3, 4, 5, 6, 7, 8, 9, 10, 11,12, 13, 14, 15, 16, 17, 18, 19, or 20 nm or any fraction of an integertherebetween), and more preferably between about 1 nm and 10 nm.

FIGS. 1(a), 1(b), 1(c), and 1(d) illustrates quantum dots according tosome embodiments of the invention. In particular, FIG. 1(a) illustratesa quantum dot 100 comprising a core 102, according to an embodiment ofthe invention. A core (e.g., the core 102) of a quantum dot may compriseinorganic crystals of Group IV semiconductor materials including but notlimited to Si, Ge, and C; Group II-VI semiconductor materials includingbut not limited to ZnS, ZnSe, ZnTe, ZnO, CdS, CdSe, CdTe, CdO, HgS,HgSe, HgTe, HgO, MgS, MgSe, MgTe, MgO, CaS, CaSe, CaTe, CaO, SrS, SrSe,SrTe, SrO, BaS, BaSe, BaTe, and BaO; Group III-V semiconductor materialsincluding but not limited to AIN, AIP, AlAs, AlSb, GaN, GaP, GaAs, GaSb,InN, InP, InAs, and InSb; Group IV-VI semiconductor materials includingbut not limited to PbS, PbSe, PbTe, and PbO; mixtures thereof; andtertiary or alloyed compounds of any combination between or within thesegroups. Alternatively, or in conjunction, a core can comprise acrystalline organic material (e.g., a crystalline organic semiconductormaterial) or an inorganic and/or organic material in eitherpolycrystalline or amorphous form.

A core may optionally be surrounded by a shell of a second organic orinorganic material. FIG. 1(b) illustrates a quantum dot 104 according toanother embodiment of the invention. Here, the quantum dot 104 comprisesa core 106 that is surrounded by a shell 108. A shell (e.g., the shell108) may comprise inorganic crystals of Group IV semiconductor materialsincluding but not limited to Si, Ge, and C; Group II-VI semiconductormaterials including but not limited to ZnS, ZnSe, ZnTe, ZnO, CdS, CdSe,CdTe, CdO, HgS, HgSe, HgTe, HgO, MgS, MgSe, MgTe, MgO, CaS, CaSe, CaTe,CaO, SrS, SrSe, SrTe, SrO, BaS, BaSe, BaTe, and BaO; Group III-Vsemiconductor materials including but not limited to AlN, AlP, AlAs,AlSb, GaN, GaP, GaAs, GaSb, InN, InP, InAs, and InSb; mixtures thereof;and tertiary or alloyed compounds of any combination between or withinthese groups. Alternatively, or in conjunction, a shell can comprise acrystalline organic material (e.g., a crystalline organic semiconductormaterial) or an inorganic and/or organic material in eitherpolycrystalline or amorphous form. A shell may be doped or undoped, andin the case of doped shells, the dopants may be either atomic ormolecular. A shell may optionally comprise multiple materials, in whichdifferent materials are stacked on top of each other to form amulti-layered shell structure.

As illustrated in FIGS. 1(c) and 1(d), a quantum dot may optionallycomprise a ligand layer comprising one or more surface ligands (e.g.,organic molecules) surrounding a core, according to some embodiments ofthe invention. In FIG. 1(c), a quantum dot 110 comprises a core 112 anda ligand layer 114 surrounding the core 112. In FIG. 1(d), a quantum dot116 comprises a core 118 and a ligand layer 122 surrounding the core118. Here, the quantum dot 116 also comprises a shell 120 surroundingthe core 118, where the shell 120 is positioned between the core 118 andthe ligand layer 122.

Optical Properties

Linear Optical Properties:

One of the most dramatic examples of “quantum confinement” effects isthat, for a semiconductor material, the energy gap shifts as a functionof size. This can be seen in FIG. 2, where the energy gap of quantumdots fabricated from silicon, referred to herein as “silicon quantumdots”, is plotted as a function of the size (e.g., diameter) of thequantum dots, according to an embodiment of the invention. The siliconquantum dots were made as described herein. The vertical axis representsthe energy gap of the silicon quantum dots, and the horizontal axisrepresents the size of the silicon quantum dots. The observed values forthe energy gap (dots with error bars) are compared againstpseudopotential and tight-binding models (solid line) and against thesimple effective mass theory (dashed line).

The same effect can be seen for the emission wavelength as a function ofthe size of quantum dots. FIG. 3 illustrates photoluminescence (PL)spectra from six samples with different sizes of silicon quantum dots,according to an embodiment of the invention. The silicon quantum dotswere made as described herein and include shells formed of an oxide. Thevertical axis represents a normalized PL signal, and the horizontal axisrepresents the emission wavelength. The PL spectra illustrated in FIG. 3is obtained by optically exciting the silicon quantum dots withultraviolet light. The wavelength of the optical excitation is shorterthan the wavelength at the absorption edge of the silicon quantum dots.FIG. 3 demonstrates the range of sizes that can be made with the methodsdescribed herein. The quantum dots shown at the top of FIG. 3 are notdrawn to scale and are meant to illustrate the relative size of thequantum dots responsible for the PL spectra. FIGS. 2 and 3 demonstratethe unprecedented control that can be obtained over absorption andemission characteristics of the silicon quantum dots.

Through a series of relations called the Kramers-Kroenig equations, theproperties of refractive index and dielectric constant can be related toabsorption. As such, size-dependent control of absorption allows controlof refractive index.

In addition to the size of a quantum dot, the optical and electronicproperties are also strongly influenced by the material from which it isfabricated. Quantum confinement effects represent a modulation of thebulk properties of the material. As such, any changes resulting from areduction in size are made relative to the bulk properties of thematerial. By selecting (e.g., independently selecting) the appropriatecombination of quantum dot size and material, an even greater control ofthe optical and electronic properties of a quantum dot is provided. Asan example, FIGS. 4(a) and (b) show the size dependent absorption andemission of germanium quantum dots, which differ from those of siliconquantum dots, according to an embodiment of the invention. The germaniumquantum dots were made as described herein. In FIG. 4(a), the verticalaxis represents the energy gap of the germanium quantum dots, and thehorizontal axis represents the size of the germanium quantum dots. Theobserved values for the energy gap (open dots with error bars) arecompared against theoretical predictions (solid dots and solid line). InFIG. 4(b), a size-selective PL spectrum is shown, where the verticalaxis represents a normalized PL signal, and the horizontal axisrepresents the emission wavelength. The far right curve is offsetvertically for clarity. The PL spectra shown in FIG. 4(b) are collectedusing different excitation wavelengths, such that only quantum dots withenergy gaps less than or equal to the photon energy of the excitationlight (i.e., greater than a certain quantum dot size) are excited.

Relation of Size and Material to Dielectric Constant and Index ofRefraction

For most materials, the index of refraction far from resonance decreasesas the energy gap of the material increases (a consequence of theKramers-Kroenig equations). This explains, for example, why the index ofrefraction of transparent materials (e.g., silica, metal halides, andorganics) is less than that for inorganic semiconductors with smallerrelative absorption energies. This effect also typically applies toquantum dots. In this case, as the size of the quantum dot decreases,the energy gap increases, decreasing the index of refraction. Thus, forquantum dots, the off-resonant index of refraction (at a fixedwavelength) typically correlates with size, affording another method tocontrol the optical properties of the quantum dots.

Relation of Concentration of Quantum Dots to Dielectric Constant andIndex of Refraction

Embodiments of the invention involve altering the index of refraction ofa material by varying the concentration of quantum dots in the material.An example of this is shown in FIG. 5(a), which illustratesconcentration dependence of the linear index of refraction of engineerednanocomposite materials doped with silicon and germanium quantum dots,according to an embodiment of the invention. The silicon and germaniumquantum dots were made in accordance with the methods described herein.The index of refraction is plotted as a function of the quantum dotconcentration expressed in weight percent. In this figure, the index ofrefraction is measured in the visible range (sodium D line).

This concentration dependence provides yet another method of controllingthe overall refractive index of a material by utilizing the propertiesof quantum dots. The ability to embed quantum dots into a variety ofhost materials will be discussed in a later section.

Nonlinear Optical Properties

In general, a wide variety of nonlinear optical phenomena can arise whenmaterials are exposed to high-intensity light. Some of these nonlinearphenomena are used in certain aspects of telecommunications (e.g., Ramanamplifiers) and many are being considered for future use (e.g.,four-wave mixing, cross-phase modulation, and solitons). Althoughnonlinear phenomena are typically associated with high-intensities,these phenomena are also observed at lower intensities due to phasematching, resonant enhancement, and/or long interaction lengths.

Light incident on a material can induce a polarization (P), which can beexpressed as (in SI units)P = ɛ₀χ  E = ɛ₀⌊χ⁽¹⁾E + χ⁽²⁾E × E + χ⁽³⁾E × E × E + …  ⌋,where E is the electric field strength, ε₀ is the electric permittivity,χ is the overall optical susceptibility, and χ^((n)) is the nth orderoptical susceptibility. Since χ⁽²⁾ phenomena are typically only presentin materials that lack inversion symmetry (e.g., non-centrosymmetry),certain embodiments of the invention primarily exploit χ⁽³⁾ phenomena,which can be exhibited by all materials. It should be recognized thattensor elements of χ⁽³⁾ are in general complex quantities. The inducedrefractive index change Δn and the nonlinear index of refraction γ arerelated to the real part of appropriate tensor elements of χ⁽³⁾, e.g.,Re[χ⁽³⁾ ₁₁₁₁], while the two-photon absorption coefficient β is relatedto the imaginary part of appropriate tensor elements of χ⁽³⁾, e.g.,Im[χ⁽³⁾ ₁₁₁₁]). In particular, certain embodiments of the inventionexploit phenomena that change the index of refraction of a material bycreating an effective optical susceptibilityχ_(eff)=χ⁽¹⁾+χ⁽³⁾ E×E=χ ⁽¹⁾+χ⁽³⁾ I,where I is the intensity of the particular light beam creating theeffective optical susceptibility (and where the higher order terms areassumed to be small and are therefore neglected here, although they canbe utilized as well), which can affect the same light beam or anotherlight beam at the same or different frequency. This leads to aneffective or overall index of refraction given byn(ω′)=n ₀+γ(ω′,ω)I(ω),and an operational definition for a nonlinear index of refraction γgiven by${\gamma\left( {\omega^{\prime},\omega} \right)} = \frac{{n\left( \omega^{\prime} \right)} - n_{0}}{I(\omega)}$where n(ω′) is the effective index of refraction at ω′, n₀ is thelow-intensity refractive index (e.g., the linear index of refraction),and I(ω) is the intensity of light with optical frequency ω that createsthe effective optical susceptibility or index change. The nonlinearindex of refraction γ(ω′,ω) is related to χ⁽³⁾ijkl(−ω′, ω′, ω,−ω), e.g.,χ⁽³⁾1111(−ω′, ω′, ω,−ω). If only one light beam is involved, then ω′ canbe set to equal to ω. If two light beams are involved, then ω′ and ω canbe the same or different. Situations where ω′ and ω are the same cancorrespond to degenerate conditions (which is further discussed herein),in which case the nonlinear index of refraction γ can be referred to asa degenerate nonlinear index of refraction (or γ_(deg)). Situationswhere ω′ and ω are different can correspond to non-degenerate conditions(which is further discussed herein), in which case the nonlinear indexof refraction γ can be referred to as a non-degenerate nonlinear indexof refraction (or γ_(nondeg)). As one of ordinary skill in the art willunderstand, an optical frequency of a light beam (e.g., ω or ω′) isinversely related to a wavelength of the light beam (e.g., λ or λ′).

This intensity dependent refractive index n(ω′) can be exploited forall-optical switching and optical signal processing. For certainapplications, nonlinear absorption processes are of particularimportance, in which case, optimization of Im[χ⁽³⁾ijkl] is preferred.

Nonlinear Optical Properties of Quantum Dots

In general, three mechanisms are principally responsible for χ⁽³⁾nonlinearities in quantum dots. These effects fall into the broadcategories of resonant, nonresonant, and near-resonant effects. Thesecategories can be further subdivided into degenerate (e.g., all lightbeams have the same wavelength) and non-degenerate (e.g., one or morelight beams have different wavelengths) cases.

1) Resonant Effects

Resonant processes typically result from a change in electronicproperties upon resonant excitation (e.g., the linear absorption oflight). This leads to a corresponding change in refractive index,following the Kramers-Kroenig relations. The magnitude of an absorptionchange, and hence the optical nonlinearity, is directly related to theground state absorption cross-section modified by any excited stateabsorption. In the case of a material with discrete states, such asmolecules or quantum dots, the optical nonlinearity results fromstate-filling and is related to (σ_(g)−σ_(e)), where σ_(g) and σ_(e) arethe absorption cross sections of the material in the ground and excitedstates respectively, with a reduction in refractive index occurring fora reduction in absorption. For quantum dots, further enhancement of χ⁽³⁾results from unique physical phenomena such as quantum confinement,local electric field effects, and quantum interference effects.

As indicated above, the optical nonlinearity is related to(σ_(g)−σ_(e)), so that increasing the oscillator strength of opticaltransitions from the ground state generally increases the opticalnonlinearity. In the case of quantum dots, a decrease in size increasesthe spatial overlap of the electron and hole wave functions, which inturn increases the oscillator strength. Resonant nonlinearity thereforetends to increase with decreasing size. This enhancement, however, canbe limited by any size dispersion.

Another important effect arises from the presence of one or more defectsin a quantum dot. Defects can be present as trap states within thequantum dot. Due to the enormous surface to volume ratio in the sizerange of quantum dots, most relevant traps exist on the surface. If notpassivated correctly, resonant excitation of a quantum dot createselectron-hole pairs that quickly relax into these surface-states. Holes,with their relatively large effective mass, tend to trap more easily,while the electrons, with their smaller effective mass, remain largelydelocalized. The result is a spatial separation of the electron and holewavefunctions and a decrease in oscillator strength, reducing themagnitude of the resulting nonlinearity. Furthermore, by tailoring therate of relaxation between the delocalized quantum dot states and thelocalized surface states, it is possible to control the response time ofthe resonant optical nonlinearity.

Resonant nonlinearities can be utilized in both the degenerate andnon-degenerate cases with respect to the wavelength of control and databeams. In the degenerate case, the wavelength range-of-interest liesnear the absorption edge. For a single beam, the absorption can besaturated, leading to an intensity dependent absorption, commonly knownas saturable absorption. For degenerate control and data beams, thecontrol beam can modulate the transmission of the data beam, leading toan optical modulator. The refractive index change caused by theabsorption change can also be utilized. Due to the broad electronicabsorption in semiconductors in general and quantum dots in particular,resonant nonlinearities can be observed for the case where the controlbeam and the data beam are non-degenerate. In this case, the controlbeam can be of higher photon energy, such that carriers are generatedwhich relax (primarily via phonon emission) towards the band edge, wherethe absorption bleaching and/or excited state absorption can affect thedata beam of lower photon energy (but still resonant).

2) Nonresonant Effects

In contrast to resonant nonlinearities, where linear absorption of lightis typically required, non-resonant nonlinearities typically do notrequire single-photon absorption of light. As a result, nonresonantnonlinearities are intrinsically fast since excited state relaxation isnot required. However, nonresonant nonlinearities are generally smallerthan resonant nonlinearities, due to the lack of strong single-photonresonance enhancement (although multi-photon resonance can be utilizedto enhance the nonresonant nonlinearity).

There are three primary enhancement factors that can be utilized fornonresonant nonlinearities in quantum dots: quantum confinement,multi-photon resonance enhancement, and local-field effects. Quantumconfinement provides an increase in oscillator strength due to enhancedwavefunction overlap (as described above), which enhances χ⁽³⁾.Multi-photon resonances can be utilized in the absence of single-photonresonances to enhance the nonresonant nonlinearity. However,multi-photon resonances can introduce unwanted nonlinear absorptivelosses. For certain applications, the ideal situation is one where therelevant light beams are just below the threshold of a multi-photonresonance, thereby allowing some resonant enhancement withoutsignificant nonlinear absorption loss. Finally, local field effects canbe utilized to enhance the nonresonant χ⁽³⁾. In particular, for ananocomposite material in which quantum dots with dielectric constant ε₁are imbedded in a matrix material with dielectric constant ε₂, anexternally applied electric field (such as that originating from anelectromagnetic light source) can be locally enhanced at the quantumdots if ε₁>ε₂, with the magnitude of the enhancement related toΔε=ε₁−ε₂. Such a situation can arise by embedding the quantum dots in alower index matrix material. When illuminated by light, the electricfield at the quantum dots is enhanced compared to the incident externalfield, in turn leading to an increase in the overall nonlinear response.This enhancement increases with size of the quantum dots as the quantumdot bandgap energy decreases, resulting in an increase in dielectricconstant

Nonresonant nonlinearities can be utilized in the non-degenerate case aswell. In this case, the control beam can have either higher or lowerphoton energy than the data beam. One advantage of the non-degeneratecase is that enhancement of cross-phase modulation (the control beaminducing an index change seen by the data beam) can occur withoutenhancement of self-phase modulation (the data beam affecting itself bythe self-induced index change), which can cause some deleterious effectsfor telecommunications data streams.

3) Near-resonant Effects

Near-resonant nonlinearities can be classified into two categories:degenerate (typically close to resonance) or non-degenerate (typicallywith one beam resonant and the other beam nonresonant). In the formercase, the beams are typically very close to the resonance edge, i.e.,just above, just below, or exactly at the edge of resonance, so thateither no direct excitation of the material occurs through linearabsorption or very little direct absorption occurs. The non-degeneratecase is perhaps the more useful situation, as the refractive indexchange induced by resonant excitation via a control beam causes a phasechange for the data beam that is below resonance (so as to minimizelosses due to single- or multi-photon absorption). For example, therefractive index change due to the absorption saturation that extends tophoton energies well below the absorption edge can be utilized, wherecarriers can be directly generated using the control beam instead ofgenerating carriers via two-photon absorption using a high-intensitydata beam. In addition, the excitation of free carriers in quantum dotsdue to absorption of control beam photons can lead to a refractive indexchange caused by other free carrier effects. For example, due to theirsmall size, quantum dots typically intrinsically have high free carrierdensities for even single photon absorption (e.g., ˜10¹⁸ carriers/cm³for one photon absorption in a single quantum dot). This leads toeffects such as quantized Auger recombination and enhanced reflectivity(due to a large plasma frequency) at high enough carrier densities(e.g., ˜10²⁰ carriers/cm³).

Size Dependence

From the discussion above, the size dependence (for a given quantum dotmaterial) of both resonant and nonresonant nonlinear processes can bederived. Typically, for resonant optical nonlinearity, the magnitude ofthe nonlinearity increases as the quantum dot size decreases, decreasesas the number of quantum dots with traps that localize electrons orholes increases, and decreases as the size dispersion increases.

Typically, for nonresonant processes, the optical nonlinearity increaseswith increasing quantum dot size, increases with increasing index ofrefraction of the quantum dot, increases with decreasing index ofrefraction of the surrounding matrix material. There is the caveat thatthese trends may not continue indefinitely to all sizes of quantum dotsbut can be useful as aids in practical design considerations. Bycarefully tailoring the specific size of the quantum dot, resonanteffects, nonresonant effects, or both, can be used to optimize theresulting nonlinear response.

Quantum Dot Material Dependence

One important consideration for a material forming a quantum dot isthat, for bound electrons, the optical nonresonant nonlinearitytypically depends on the energy gap of the material as 1/E_(g) ^(n),where n typically ranges from about 4 to about 6. The nonresonantnonlinearity therefore can increase significantly as the energy gapdecreases. This trend favors a combination of large quantum dot sizesand materials with intrinsically small bandgap energies. At the sametime, however, the photon energy in the wavelength range-of-interest canaffect the choice of material and quantum dot size in order to avoidsignificant linear and nonlinear absorption. Specifically, the materialin the bulk form desirably should have an energy gap roughly equal to orgreater than the photon energy in the wavelength range-of-interest for adata beam in order to exploit quantum confinement effects that shift theenergy gap to higher energies. At the same time, to avoid significantmulti-photon absorption effects, the energy gap of the materialdesirably should be sufficiently large that the energy gap of theresulting quantum dot is greater than two times the photon energy of thedata beam photons.

For the case of nonresonant optical nonlinearities, these two concernsspecify opposing trends that bracket the energy gap of the material ofchoice for quantum dots according to some embodiments of the presentinvention. The material in the bulk form desirably should have an energygap less than this bracketed energy in order to exploit quantumconfinement effects that shift the energy gap to higher energies. As anexample, to avoid two-photon losses in degenerate all-optical switchingcomponents operating near 1550 nm (corresponding to a photon energy of0.8 eV) and to also take advantage of the 1/E_(g) ^(n) behavior of thenonlinear response, the quantum dot energy gap should be less than butclose to 775 nm (or greater than but close to 1.6 eV).

Enhanced Optical Properties

In addition to size-dependent spectral characteristics, quantumconfinement can also result in an enhancement in the magnitude ofvarious optical and electronic properties due to a redistribution of thedensity of states. Properties such as absorption cross-section andexcited-state polarizability have been found to be enhanced by severalorders of magnitude over bulk materials. χ⁽³⁾ can also be enhanced byquantum confinement, as described previously.

Additional Effects

There are many practical definitions of a figure-of-merit (FOM) thattake into account the many parameters that can be important and relevantfor all-optical switching. One example of such a FOM is defined asΔn/α·τ, where Δn is the induced refractive index change, α is the linearand nonlinear absorption coefficient, and τ is the response time of thematerial. For this FOM, which is particularly relevant for resonantoptical nonlinearities where light absorption is used, the larger theFOM, the better will be the performance of the all-optical switching. Adefinition of a FOM useful for nonresonant optical nonlinearities, whereideally no or little light absorption occurs, is 2γ/βλ, where γ is thenonlinear index of refraction, β is the two-photon absorptioncoefficient, and λ is the wavelength of operation. In this case, usefulall-optical switching typically occurs when FOM>1. According to someembodiments of the invention, the following effects can be important forthe formation of nanocomposite materials with a FOM in a usable rangefor practical optical switching.

The Effect of Defects on FOM:

Defects within quantum dot materials can have a substantial negativeimpact on their performance as nonlinear optical materials. Defects inthe core and/or surface of the quantum dot can yield direct absorptionof below-bandgap photons, increasing optical losses, and decreasing theoverall FOM. As a result, while χ⁽³⁾ may be high, the material can stillbe inappropriate for optical switching. The effect of defects on opticalswitching using quantum dots has not been previously considered asdiscussed herein.

One important aspect of some embodiments of the invention is that, forquantum dots to be used as a nonlinear optical material, they desirablyshould comprise a substantially defect-free core. In this case, the term“defect” typically refers to defects with energy below the energy gap ofthe quantum dot core or within the energy range of the wavelengthrange-of-interest. Additionally, the surface of quantum dots should bewell passivated, such that there are substantially no defect states.Passivation can be accomplished, for example, through the inclusion ofappropriate surface ligands in the ligand layer to bind to defect sitesand remove them from the energy gap. Alternatively, or in conjunction,passivation can be achieved by applying a shell to the quantum dot coreto fill or eliminate the defect sites. In this case, the shell materialis preferably a material with an energy gap that is higher than thatcorresponding to the wavelength range-of-interest, and more preferablyhigher than the energy gap of the quantum dot core. Additionally, theshell desirably should be substantially defect-free or should havedefects that can be eliminated through the inclusion of appropriatesurface ligands.

Concentration Effects:

One important aspect of some embodiments of the invention is that thenonlinear properties of a material including quantum dots can besubstantially affected by correlated interactions between two or morequantum dots. In particular, while χ⁽³⁾ can be proportional toconcentration of quantum dots at low concentrations, as theconcentration increases, the individual quantum dots can get closeenough to interact with each other, producing collective phenomena thatcan further enhance nonlinearity. This effect is seen in FIG. 5(b),which illustrates concentration dependence of the optical nonlinearityof engineered nonlinear nanocomposite materials doped with silicon andgermanium quantum dots, according to an embodiment of the invention. Thesilicon and germanium quantum dots were made in accordance with themethods described herein. The vertical axis represents the nonlinearindex of refraction γ, and the horizontal axis represents the relativeconcentration of quantum dots in a matrix material. As shown in FIG.5(b), γ can increase superlinearly with concentration at sufficientlyhigh concentrations. The effect of concentration (and particularly thesuperlinear concentration dependence) on optical switching using quantumdots has not been previously considered as discussed herein.

For FIG. 5(b), γ arises as a result of nonresonant degeneratenonlinearities. The values attained for γ are particularly large. Asshown in FIG. 5(b), the nanocomposite material doped with siliconquantum dots has γ as high as about 8×10⁻⁵ cm²/W, which is 9 orders ofmagnitude larger than the bulk material from which the silicon quantumdots are fabricated (bulk silicon has a nonresonant degenerate γ ofabout 8×10⁻¹⁴ cm²/W). Additional nonlinear enhancement can be inducedthrough the appropriate selection of molecular species in the ligandlayer (see discussion below on Molecular Tethers).

Summary of Nonlinear Optical Properties of Quantum Dots

Enhancement and tunability of the optical nonlinearity in individualquantum dots and multi-quantum dot nanocomposites, combined withsubstantially defect free and/or well passivated quantum dot cores,provide the engineered nonlinear nanocomposite materials according tosome embodiments of the current invention. Such nanocomposite materialscan satisfy various characteristics for an ideal χ⁽³⁾ based opticalmaterial that include (but are not limited to): large Re[χ⁽³⁾_(ijkl] in the wavelength range-of-interest; a multi-photon transition that can be tuned to maximize near-resonance enhancement while minimizing optical loss due to absorption; the use of non-degenerate control and data beams where the control beam is resonant and induces a large index change at the data beam wavelength while introducing low optical loss at that wavelength; the use of degenerate control and data beams to allow cascading of devices; and low optical loss due to absorption by defects.)

Colloidal Quantum Dots

Structures comprising quantum dots can be fabricated using vapordeposition, ion-implantation, photolithography, spatially modulatedelectric fields, semiconductor doped glasses, strain-induced potentialvariations in quantum wells, atomic width fluctuations in quantum wells,and a variety of other techniques. Preferably, quantum dots are formedor used in a form that can be easily incorporated into flexible orengineered optical materials or devices. In addition, it is desirable toseparate the optical properties of the quantum dots from those of amatrix material to achieve a sufficiently large FOM with reducedabsorption and/or scattering by the matrix material.

In a preferred embodiment, the current invention comprises colloidalquantum dots. Colloidal quantum dots are freestanding nanostructuresthat can be dispersed in a solvent and/or a matrix material. Suchcolloidal quantum dots are a particularly preferred material for someembodiments of the current invention because they can be more easilypurified, manipulated, and incorporated into a matrix material.

It will be apparent to one of ordinary skill in the art that thedefining characteristic for a “colloidal” quantum dot is that it is afreestanding nanostructure. The method of fabrication, size, and shapeof the particular colloidal quantum dot do not bear on itsclassification.

Chemical Properties

Chemically Controllable Surface

According to some embodiments of the invention, a unique physicalcharacteristic of quantum dots is that, while the core can comprise acrystalline semiconductor material, the surface can be coated with avariety of different organic and/or inorganic materials. These surfacecoatings (e.g., shells or ligand layers) can impart stability andchemical activity, as well as passivation of electrically and opticallyactive defect sites on the quantum dot surface. These surface coatingsare optionally substantially different in chemical nature than theinorganic core. As a result, while quantum dots can comprise primarily ahighly nonlinear semiconductor material, they substantially appear tothe surrounding material as surface ligands. As such, the processabilityand chemical stability of this highly nonlinear and tunable opticalmaterial can primarily be a function of the surface layer and not afunction of the material that provides the majority of the opticalcharacteristics.

Surface ligands are preferably bi-functional. By bi-functional, it ismeant that there are at least two portions of the surface ligand suchthat one portion interacts primarily with the quantum dot surface, whilethe second portion interacts primarily with the surrounding environment(e.g., solvent and/or matrix material). These at least two portions ofthe surface ligand may be the same or different, contiguous ornoncontiguous, and are optionally contained within two or more differentmolecular species that interact with each other to form the ligandlayer. The at least two portions can be selected from a group consistingof hydrophilic groups, hydrophobic groups, or amphiphilic groups. Theinteraction of each of the at least two portions and the quantum dot orsurrounding environment can be covalent or noncovalent, stronglyinteracting or weakly interacting, and can be labile or non-labile. Theat least two portions can be selected independently or together.

In some embodiments of the current invention, the surface ligands areselected such that the portion that interacts with the quantum dotpassivates defects on the surface such that the surface is madesubstantially defect-free. At the same time, the portion that interactswith the environment is selected specifically to impart stability andcompatibility (e.g., chemical compatibility or affinity) of the quantumdot within a matrix material that is selected for a specificapplication. Simultaneously satisfying both of these requirements is animportant aspect of certain embodiments of the current inventionrelating to the development of an engineered nonlinear nanocompositematerial. Alternative methods of achieving these requirements include(but are not limited to): 1) Passivating the surface of the quantum dotindependent of the ligand layer (e.g., using a shell or creating anintrinsically defect free surface), while the environmentalcompatibility is imparted by the surface ligands, or 2) imparting bothpassivation and environmental compatibility independent of the ligandlayer. Achieving passivation of the surface of quantum dots is oneadvantage of using colloidal quantum dots over alternate approaches.

Through the appropriate selection of surface ligands, quantum dots canbe incorporated into a variety of matrix materials such as, for example,liquids, glasses, polymers, crystalline solids, and even close-packedordered or disordered quantum dot arrays. The resulting nanocompositematerials can be formed into homogeneous, high-quality optical films ofquantum dots. Alternatively, the chemistry can be selected to allowdispersion of the quantum dots into a matrix material with acontrollable degree of aggregation, forming micron or sub-micron sizedclusters. The result is an increased local fill-factor and an enhancedlocal field effect that may further increase the nonlinear response ofthe nanocomposite materials of embodiments of the present invention.

An important aspect of some embodiments of this invention relates toeffectively separating the optical properties of the quantum dots fromthe optical, chemical, mechanical, and other properties of the matrixmaterial. In this aspect, it is possible to combine the largenonlinearities of quantum dots with the ease of handling andprocessability of a matrix material such as a standard polymer. Thus,this aspect provides two additional features of an ideal χ⁽³⁾ basedoptical material: physical and chemical compatibility with specificdevice architectures and the ability to be easily processed forincorporation.

Molecular Tethers:

In addition to conveying stability and chemical compatibility with thesurrounding environment, the ligand layer can optionally be used totailor the physical, optical, chemical, and other properties of thequantum dots themselves. In this case, it is not just the chemicalnature of the surface ligand but also the interaction of the surfaceligand with the quantum dot that imparts an additional level of controlover the physical, optical, chemical, and other properties of theresulting nanocomposite material. We refer herein to any molecule,molecular group, or functional group coupled (e.g., chemically attached)to the surface of a quantum dot that imparts additional functionality tothe quantum dot as a “molecular tether”. In some cases, the moleculartether can be electrically active, optically active, physically active,chemically active, or a combination thereof. The inclusion of moleculartethers into a quantum dot structure is an important aspect of someembodiments of the present invention.

Active species are used to precisely control the electrical, optical,transport, chemical, and physical interactions between quantum dots andthe surrounding matrix material and/or the properties of individualquantum dots. For instance, a conjugated bond covalently bound to thesurface of one or more quantum dots may facilitate charge transfer outof one quantum dot and into another. Similarly, a physically rigidactive group bound in a geometry substantially normal to the surface ofa quantum dot can act as a physical spacer, precisely controllingminimum interparticle spacing within an engineered nonlinearnanocomposite material.

As described above, collective phenomena (e.g., at high concentrations)are an important aspect of some embodiments of the current invention.This aspect can be further enhanced by allowing individual quantum dotsto interact with one another using molecular tethers that fosterinteractions between quantum dots. At sufficiently high numberdensities, the molecular tethers begin to make contact with moleculartethers from other quantum dots or with other quantum dots directly.This can serve to augment nonlinearity by controlling the interactionbetween quantum dots and thus increasing the degree of collectivephenomena compared to single particle phenomena. Molecular tethers mayinclude, but are not limited, to conducting polymers, charge transferspecies, conjugated polymers, aromatic compounds, or molecules withdonor-acceptor pairs. These molecular tethers can foster electrondelocalization or transport and thus can increase the interactionbetween quantum dots. Additionally, the molecular tethers can beselected to facilitate high quantum dot number densities without thedetrimental aggregation that often plagues high concentration systems.

Molecular tethers can also be selected to impart stability of quantumdots under a variety of environmental conditions including ambientconditions. Molecular tethers can optionally contain chemically activegroups to allow quantum dots to be attached to polymer backbones, alongwith other active molecules. This provides a method for controlling thedensity of quantum dots within close proximity of molecules thatinfluence a variety of functions such as carrier transport ordelocalization.

An additional aspect of the present invention is the use of moleculartethers to physically connect two or more quantum dots in a 1dimensional, 2 dimensional, or 3 dimensional structure or array. Suchquantum dot superstructures can be created to initiate multiple dotquantum interference interactions or collective phenomena yielding newand useful properties such as enhanced, nonsaturating opticalnonlinearities. The length and properties of these molecular tethers canbe tailored to enhance or generate specific quantum phenomena. Thesenanostructures can have the properties of single quantum dots or anensemble of quantum dots depending on the nature of the moleculartethers. For certain applications, more than one type of moleculartether can be used to connect quantum dots.

The quantum dots according to some embodiments of the inventionexemplify microscopic conditions that enhance the nonresonant opticalnonlinearity arising from local electric field effects described above.Whether the quantum dot surface is terminated with oxide or ligand layer(e.g., molecular tethers), the result is a particle (e.g., a core of thequantum dot) with dielectric constant ε₁ surrounded by an environment(e.g., the surface oxide layer or molecular tethers) with dielectricconstant ε₂ where ε₁>ε₂. Therefore, the enhancement of the nonresonantoptical nonlinearity can be engineered by the judicious choice of oxideor molecular tether without resorting to a surrounding bulk matrixmaterial. In other words, a single quantum dot as described in thispatent should exhibit an enhanced nonresonant optical nonlinearity sincethe surface layer functions as a surrounding matrix material with alower dielectric constant. Optionally, molecular tethers can be used toconnect quantum dots together without a separate matrix material. Inthis case, an extrinsic matrix material is not required since theindividual interconnected quantum dots exhibit an enhanced localelectric field effect.

A preferred approach of attaching appropriate molecular tethers to aquantum dot surface can be thought of as essentially treating a quantumdot as a very large molecule (e.g., a macro-molecule) and the moleculartethers as functionalizations of this large molecule. This creates alarge three-dimensional structure with enhanced nonlinear opticalproperties resulting from the combination of quantum effects from thequantum dot and carrier polarization and delocalization effects from themolecular tethers and from the interaction of these two effects. Theseproperties can be tailored by the choice of molecular tethers. Inaddition, a quantum dot can also represent a large and stable reservoirof polarizable charge that also contributes to a large nonlinear opticalresponse.

Macroscopic Quantum Dot Solids

Macroscopic solids can be fabricated in which quantum dots form asubstantially close-packed array (e.g., a cubic closed-packed array) inthe absence of an extrinsic matrix material. These “quantum dot solids”can either be crystalline, polycrystalline, or amorphous. Whilecontaining a relatively high density of quantum dots, quantum dot solidscan still be easily processed since, during formation, the quantum dotscan be dispersed in a solvent that is subsequently removed. Uniformsolid quantum dot films, for instance, can be formed using standardspin-coating techniques as, for example, described in C. R. Kagan etal., “Long-range resonance transfer of electronic excitations inclose-packed CdSe quantum-dot solids,” Phys. Rev. B 54, 8633 (1996), thedisclosure of which is incorporated herein by reference in its entirety.In addition, surface ligands can still be selected to impart solventcompatibility and appropriate chemical stability to the final quantumdot solid. In contrast to the interconnected material described above,these macroscopic quantum dot solids are typically not held together bymolecular bonds but rather by Van der Waals forces.

High-quality optical materials can be fabricated from quantum dot solidswith substantially homogeneous optical properties throughout thematerial. The density of quantum dots can be tuned by modifying thelength and/or structure of the surface ligands. Careful selection ofsurface ligands can produce continuously tunable densities up to amaximum fill-factor of about 75% by volume of the quantum dot solid,preferably between about 0.005% and 75% by volume (e.g., between about10% and 75% by volume, between about 30% and 75% by volume, betweenabout 50% and 75% by volume, or between about 60% and 75% by volume).The surface ligands are optionally removed partially or completely byheating or chemical treatment after the quantum dot solid is formed.More specifically, the length of the surface ligands can be used todefine the spacing between quantum dots. By combining the ability tocreate density-controlled quantum dot solids with variable densityquantum dots in a matrix material, the concentration of quantum dots,and therefore the nonlinear index of refraction of the materialsdescribed herein, can be tuned over many orders of magnitude.

In the case of quantum dot solids, the surface ligands can take theplace of an extrinsic matrix material according to some embodiments ofthe current invention. In the case of close-packed quantum dots in whichthe surface ligands have typically been removed, the quantum dotsthemselves are considered to form their own “intrinsic” matrix material.Quantum dot solids according to some embodiments of the invention can befabricated in a variety of ways, such as, for example, described in C.B. Murray et al., “Self-Organization of CdSe Nanocrystallites intoThree-Dimensional Quantum Dot Superlattices,” Science 270, 1335 (1995),C. R. Kagan et al., “Long-range resonance transfer of electronicexcitations in close-packed CdSe quantum-dot solids,” Phys. Rev. B 54,8633 (1996), and U.S. Pat. No. 6,139,626 to Norris et al., entitled“Three-dimensionally patterned materials and methods for manufacturingsame using nanocrystals” and issued on Oct. 31, 2000, the disclosures ofwhich are incorporated herein by reference in their entirety. Quantumdot solids according to some embodiments of the invention can befabricated with a variety of different quantum dot materials, sizes, andsize distributions. It is also possible to form mixed quantum dot solidscomprising a plurality of quantum dot materials, sizes, and sizedistributions.

Engineerable Nonlinear Nanocomposite Materials

One embodiment of the present invention comprises an engineerednonlinear nanocomposite material that combines the large nonlinear andsize dependent optical properties of quantum dots with theprocessability and chemical stability of a matrix material and/or achemically controlled quantum dot surface. By separately selecting thesize and material of the quantum dot, the surface ligands, the matrixmaterial, and the density of quantum dots within the matrix material,one can independently tune various significant characteristics indesigning an ideal nonlinear optical material.

In particular, this embodiment of the present invention comprises thefollowing characteristics that, taken together, in part or in whole,provides a substantially improved nonlinear optical material over whatis known in the art.

The effects of quantum confinement and the specific selection of quantumdot material is used to create extremely large optical nonlinearities,specifically Re[χ⁽³⁾_(ijkl], in the data beam wavelength range-of-interest, while the energies of single- and multi-photon absorption features are selected to minimize absorptive loss of the data beam and heating and to optimize resonant enhancement effects. This optimization can include the use of appropriately chosen non-degenerate control and data beams. Alternatively, the nonlinear absorption mechanisms can be enhanced, e.g., Im[χ)⁽³⁾ _(ijkl] can be optimized, depending upon the application.)

The matrix material is selected, independent of the quantum dot materialand size, With the desired chemical and mechanical properties to impartphysical and chemical compatibility with the specific devicearchitecture and materials as well as the process of incorporation intodevices.

The surface ligands of the quantum dots are selected to facilitatehomogeneous incorporation of the quantum dots into the selected matrixmaterial and are optionally selected to facilitate controlledaggregation of quantum dots within the selected matrix material.

The density of quantum dots in the matrix material is selected toprecisely tune the linear index of refraction to match the boundaryconditions for a given device architecture (in the case of high-indexmaterials, a quantum dot solid can be used).

EXAMPLE 1

This example describes a preferred embodiment in which an engineerednonlinear nanocomposite material is incorporated into a nonlineardirectional coupler that utilizes nonresonant or near-resonantnonlinearities. In the current example, the waveguide core is fabricatedfrom doped silica with an index of refraction of 1.52 at 1.55 μm. Itwill be recognized by one of ordinary skill in the art that doped silicacan have an index of refraction over a wide range of values. The currentexample is not meant to limit the scope of the invention, and it will beunderstood that variations on this example can extend to waveguide coreswith an arbitrary index of refraction.

In the case of a nonlinear directional coupler, light is evanescentlycoupled between two waveguide cores such that a signal entering onewaveguide core oscillates between the two as a function of theinteraction length. By choosing an appropriate length, the light can becoupled completely into one or the other of the two waveguide cores(i.e., the “off” state can be transmission through one or the otherwaveguide core by appropriate device design). By changing the index ofrefraction between the waveguide cores, it is possible to switch theoutput waveguide core from the “off” state to the other waveguide core(i.e., the “on” state) for a fixed length device. An index change from aχ⁽³⁾ based nonlinear material can yield extremely fast opticalswitching. However, so far no single material has been appropriate for acommercial optical switch based on a nonlinear direction coupler.

The active material in this optical device desirably should have a largenonlinear response in the data beam wavelength range-of-interest. It isalso desirable (primarily for nonresonant nonlinearities) to maximizeresonant enhancement, while simultaneously avoiding significantsingle-or multi-photon absorption. At the same time, the linear index ofrefraction of the active material desirably should be less than that ofthe core material and be close to that of the rest of the cladding toavoid disruption of the optical mode as light is guided into the activeregion.

In this example, depicted in FIGS. 6(a) through 6(e), the devicecomprises doped silica waveguide cores 602 and 604 (n=1.552) fabricatedon a doped silica substrate 606 (n=1.515). As shown in FIG. 6(b), theother three sides of the waveguide cores 602 and 604 are initiallysurrounded by air (n=1), as is the space between the waveguide cores 602and 604 in the interaction region. The space around the waveguide cores602 and 604 is then filled with an engineered nonlinear nanocompositematerial 608 (n=1.515) to match the waveguide boundary conditions of thesubstrate 606, as shown in FIG. 6(c). By illuminating the interactionregion with trigger-pulses as shown in FIGS. 6(d) and 6(e), the index ofrefraction between the waveguide cores 602 and 604 is changed,activating the switch.

Operation of this switch is slightly different than what is commonlydescribed in the art. It is best understood by presupposing that thedirectional coupler length is chosen such that in the inactivated state,the two waveguide cores 602 and 604 exchange energy such that eachoutput will receive substantially half of the power from each input(acting as a 3 dB coupler). If the illumination is such that the indexof refraction increases in the nonlinear nanocomposite material 608, theinteraction between the two propagating waveguide cores 602 and 604 willdecrease, leading to a reduction in the data energy transferred betweenthe cores 602 and 604, forcing the switch closer to a bar state. If theillumination is such that the index of refraction decreases in thenonlinear nanocomposite material 608, the interaction between the twocores 602 and 604 increases, increasing the energy transferred betweenthe cores 602 and 604, forcing the switch closer to a cross state. Oneskilled in the art will recognize that this transfer function is cyclicand that further reduction of the index of refraction of the nonlinearnanocomposite material 608 will result in oscillations between the crossand bar states. If desired, the length of nonlinear directional couplermay be chosen to include several oscillations in the inactive state,leading to an effective bias in the total oscillations.

The engineered nonlinear nanocomposite material 608 for this examplecomprises silicon oxide coated silicon quantum dots ororganic-terminated silicon or germanium quantum dots dispersed in apoly(methyl methacrylate) polymer matrix material (PMMA; n=1.49). PMMAis chosen here due to its desirable optical properties for use in the1.55 μm range and its ease of processing in waveguide structures.Examples of these desirable optical properties include high opticaltransmissivity in the visible wavelength, relatively low absorption near1550 nm, and low birefringence (as low as 0.0002 at 1550 nm has beenobserved).

In order to optimize degenerate nonresonant switching at 1.55 μm,silicon quantum dots with a diameter of around 4 nm are used, placingthe 2-photon absorption peak at higher energy than the spectral energyrange-of-interest. This is sufficient to minimize 2-photon absorptionthat may result in signal loss and heating, while maintaining asignificant resonance enhancement at the wavelength of the triggerpulse. This particular combination of quantum dot material and size alsoyields a maximum in χ⁽³⁾ at 1.55 μm. To maximize near-resonantswitching, the appropriate choice of control wavelength and quantum dotresonance at the control wavelength desirably should be chosen thatminimizes or reduces absorption loss at the data wavelength.

To facilitate incorporation of the quantum dots into PMMA, the siliconor germanium quantum dots can be coated with a ligand layer comprising along-chained hydrocarbon with a methacrylate functional group at theend. Alternatively, any functional group compatible with PMMA can beused. Quantum dots and PMMA are dissolved in an organic solvent, such astoluene, and applied to the device as shown in FIG. 6(c). Theconcentration of PMMA is determined based on the desired thickness ofthe final nanocomposite material and the method of application. In thecase of spin-coating, a 5% PMMA solution is appropriate. Theconcentration of quantum dots is selected such that the finalnanocomposite material, after deposition, has a linear index ofrefraction of 1.515. This is determined by calibrating the initialconcentration of quantum dots (as measured by the absorptioncharacteristics) to the final index of refraction of a PMMA-quantum dotfilm deposited in the method to be used. The linear index of the filmcan be measured using ellipsometry or the like.

After spin-coating the polymer-quantum dot solution over the device, thesolvent is allowed to evaporate, leaving an engineered nonlinearnanocomposite coated device as shown in FIG. 6(c). The index ofrefraction around all sides of the waveguide cores 602 and 604 ismatched and optimized for the specific device. At the same time, χ⁽³⁾and the resonance conditions for 1.55 μm are independently tuned foroptimum switching performance. As a final aspect of the current example,based on the known intensity of the trigger-pulse and the resultingnonlinear response of the engineered nonlinear nanocomposite material608, the active length of the device is selected to provide optimalswitching performance. This can be done by limiting the illuminationarea of the trigger-pulse to define the active area as in FIG. 6(d) orby designing the specific waveguide structure with the appropriateinteraction length as in FIG. 6(e). The actual active length can bedetermined empirically or through simulation.

By increasing the index of refraction of waveguide cores, substantiallylarger concentrations of quantum dots can be incorporated into theactive material while retaining functionality of the switch. This canyield substantially higher switching efficiency. For example, as shownin FIG. 6(f), with silicon waveguide cores 610 and 612 having an indexof refraction of ˜3.4, an active material 614 desirably should have anindex of refraction equal to or less than 3.39 to achieve efficientwaveguiding through the active region. This allows densities of quantumdots as high as those of close-packed quantum dot solids (eithercrystalline or amorphous).

EXAMPLE 2

To highlight the flexibility of embodiments of the current invention,this example describes a second preferred embodiment in which anengineered nonlinear nanocomposite material may be used in a waveguidenonlinear Mach-Zehnder (MZ) interferometer. In this case, as shown inFIGS. 7(a) through 7(f), a waveguide core is fabricated from partiallyoxidized silicon with an index of refraction of 2.4 at 1.55 μm. Onceagain, it will be apparent to one of ordinary skill in the art thatpartially oxidized silicon can have a range of indices of refraction,and that 2.4 is not meant to limit the scope of the invention.Variations on this example comprising other possible indices can be useddepending on the specific application.

In the nonlinear MZI of the present example, a data signal travelingalong a waveguide core is split into two separate and uncoupledwaveguide arms with a defined phase relation between them. The signalstravel along the arms for a predetermined length and are thenrecombined. Phase differences resulting from the propagation of thelight in each arm result in constructive or destructive interference ofthe signals in the output waveguide core. By modulating the index ofrefraction of one or both of the arms, the output signal can be switchedon or off by creating a relative 0- to π-phase shift between thesignals. One of ordinary skill in the art will realize that furtherchanges in index of refraction will result in cyclic exchange betweenthe on and off states. An index change from a χ⁽³⁾ based nonlinearmaterial would yield extremely fast optical switching; however, so farno single material has been appropriate for a commercial switch based onthis device.

As with the example above, the active material in this device desirablyshould have a high nonlinear response in the wavelengthrange-of-interest and no significant absorption. In this case, however,the nonlinear material is incorporated directly into the waveguide core.As such, the index of refraction of the engineered nonlinearnanocomposite desirably should be greater than that of the claddingmaterial and be close to that of the core to avoid disruption of theoptical mode as light moves into the active region.

In this example, as shown in FIG. 7(a), the device comprises a partiallyoxidized waveguide core (n=2.4) fabricated on a silica substrate(n=1.45) and surrounded by a silica cladding on three sides. The top ofthe waveguide core is bounded by air (n=1). A section of one of thewaveguide arms is etched away as shown in FIG. 7(b), filled with anengineered nonlinear nanocomposite material (n=2.4) as shown in FIG.7(c) to match the boundary conditions of the waveguide core, and thenpolished as shown in FIG. 7(d). By illuminating the active region withtrigger-pulses as shown in FIGS. 7(e) and 7(f), the index of refractionin one arm is changed, thus activating the switch. A preferredengineered nonlinear nanocomposite material for this example comprisessilicon oxide coated silicon quantum dots formed into a close-packedquantum dot solid with index of refraction tuned to 2.4.

In order to optimize switching at 1.55 μm, silicon quantum dots with adiameter of 4 nm are used, placing the 2-photon absorption peak athigher energy than the spectral energy range-of-interest. This issufficient to eliminate or reduce 2-photon absorption that may result insignal loss and potential heating of the device by the trigger-pulse.This particular combination of material and size also yields a maximumin χ⁽³⁾ at 1.55 μm. To maximize near-resonant switching, the appropriatechoice of control wavelength and quantum dot resonance at the controlwavelength desirably should be chosen that minimizes or reduces anyabsorption loss at the data wavelength.

In order to achieve precise index of refraction control within thewaveguide arm, surface ligands desirably should be selected to yield aspecific particle-to-particle spacing within the final quantum dotsolid. This can be achieved by measuring the index of refraction of manythin-films, formed by the method to be used, with quantum dotscomprising different types of surface ligands. By using ellipsometry orthe like, the index of refraction resulting from each type of surfaceligand and deposition method can be determined and calibrated fordetermining the optimum conditions for the final device deposition. Inthe case of the present example, an index of 2.4 corresponds roughly toa packing density of 70% by volume. A short-chained hydrocarbon ispreferable in this case, such as a butyl- or other alkyl group.

The quantum dots, in a solvent of hexane or toluene, are spin-coatedover the surface of the device, filling the open region of the waveguidearm as shown in FIG. 7(c). A slow spin speed is preferable, since thethickness of the material in the waveguide arm can be controlled bypolishing the overflow off the surface (1000 rpm). The concentration ofquantum dots in the solution should be high, preferably in the range of1 nM to 1 M, more preferably 10 μM to 1 mM.

After spin-coating, the solvent is allowed to evaporate, creating aclose-packed quantum dot solid filling the open region of the waveguidearm as shown in FIG. 7(c). The surface is then polished to provide anoptical-quality interface on the topside of the device in the activeregion as shown in FIG. 7(d). The index of refraction of the engineerednonlinear nanocomposite is matched to that of the waveguide core of thearm and optimized for the specific device. At the same time, χ⁽³⁾ andthe resonance conditions for 1.55 μm are independently tuned for optimumswitching performance. As a final aspect of the current embodiment,based on the known intensity of the trigger-pulse and the resultingnonlinear response of the engineered nonlinear nanocomposite material,the active length is selected to provide optimal switching. This can bedone by designing the etched length of the waveguide arm to the desiredactive length as in FIG. 7(e) or by limiting the illumination area ofthe trigger-pulse as in FIG. 7(f). The specific active length can bedetermined empirically or through simulation.

Alternatively, a nonlinear MZ interferometer can be fabricated withoutetching a portion of a waveguide core as shown in FIGS. 8(a) through8(d). In this case, a engineered nanocomposite material can be simplycast on top of the entire device as shown in FIG. 8(b) with any excessremoved as shown in FIG. 8(c), such that the active material is inevanescent contact with the signal passing through each of the arms (aswell as elsewhere). By illuminating a portion of one or both arms, theactive region can be defined as shown in FIG. 8(d). In this preferredembodiment, the engineered nonlinear nanocomposite desirably should bedesigned to have an index of refraction that is compatible withwaveguiding in the partially oxidized silicon core (e.g., n<2.4). Again,this nanocomposite material is preferably a close-packed quantum dotsolid.

Had further chemical processing steps been required in either of theabove examples, it would also be possible to select the matrix materialand/or surface ligands to impart stability of the engineered nonlinearnanocomposite under the required conditions.

The current embodiments not only provide a nonlinear material with adramatically increased nonlinear response for use in these opticaldevices, they simultaneously provide materials that have been engineeredto have optimum linear index of refraction, 2-photon absorption,near-resonance enhancement, and processability for each application.This level of independent control of optical, chemical, and mechanicalproperties does not exist in other materials.

Preferred Quantum Dot Materials

Preferred quantum dots according to some embodiments of the presentinvention comprise substantially defect free quantum dots with awell-passivated surface. Preferred quantum dots also comprise a bandgapenergy that is preferably greater than the photon energyrange-of-interest (e.g., for the data beam), and more preferably greaterthan twice the photon energy range-of-interest (primarily fornonresonant nonlinearities) for its intended applications. Whilemaintaining these requirements, the material and size of the quantumdots can be interchangeable. The specific material and size can beselected as necessary to engineer the optical characteristics for aparticular application. The following provides certain preferredcharacteristics according to some embodiments of the invention:

Core-shell Quantum Dots:

Core-shell quantum dots are particularly preferred because defects canresult in traps for electrons or holes at the surface of a quantum dotcore. These traps can degrade the electrical and optical properties ofthe quantum dot, yielding low-energy states within the bandgap of thematerial. An insulating layer at the surface of the quantum dot coreprovides a rise in the chemical potential at the interface, which caneliminate energy states that serve as traps. Surprisingly, these trapstates can actually interfere with efficient switching or decrease theFOM of a material by contributing to single or multi-photon absorption.Additionally, shells act to physically protect the core material fromchemical interactions such as oxidation, reduction, or dissolution. Forinstance, one embodiment of the present invention relates to the use ofa shell to stabilize intrinsically unstable silicon or germanium quantumdots. Optionally, the shell can provide an appropriate chemical surfacefor covalent or non-covalent binding of molecules to the quantum dot,wherein the core material may or may not provide an appropriate surfacefor such binding.

Preferably, a quantum dot will be substantially defect free. Bysubstantially defect free, it is typically meant that within the quantumdot there is fewer than. 1 defect per quantum dot, preferablysubstantially fewer than 1 defect per quantum dot, more preferably lessthan 1 defect per 1000 quantum dots, more preferably less than 1 defectper 10⁶ quantum dots, more preferably less than 1 defect per 10⁹ quantumdots. Typically, a smaller number of defects within a quantum dottranslates into an increased photoluminescence quantum efficiency. Forcertain embodiments of the invention, a quantum dot that issubstantially defect free will typically exhibit photoluminescence witha quantum efficiency that is greater than 6 percent, preferably greaterthan 10 percent, more preferably at least 20 percent, more preferably atleast 30 percent, more preferably at least 40 percent, and morepreferably at least 50 percent.

Preferably, the core will be substantially crystalline and besubstantially defect-free. By substantially defect free, it is typicallymeant that within the core there is fewer than 1 defect per quantum dot,preferably substantially fewer than 1 defect per quantum dot, morepreferably less than 1 defect per 1000 quantum dots, more preferablyless than 1 defect per 10⁶ quantum dots, more preferably less than 1defect per 10⁹ quantum dots.

In a similar manner, the shell and/or the interface region preferablywill be substantially defect free, where it is typically meant thatwithin the shell and/or the interface region there is fewer than 1defect per quantum dot, preferably substantially fewer than 1 defect perquantum dot, more preferably less than 1 defect per 1000 quantum dots,more preferably less than 1 defect per 10⁶ quantum dots, more preferablyless than 1 defect per 10⁹ quantum dots.

Size and Size-distribution:

Another preferred characteristic of the quantum dots of some embodimentsof the present invention is such that a figure-of-merit (FOM) forall-optical switching or processing can be largely insensitive to sizedispersion, contrary to results and predictions in the literature. FIG.9 illustrates a figure-of-merit (FOM) for all-optical switching with anengineered nonlinear nanocomposite material as a function of quantum dotsize, according to an embodiment of the invention. Here, thenanocomposite material includes germanium quantum dots made with methodsdescribed herein. The FOM in this case is defined as 2γ/βλ, which isapplicable for nonresonant nonlinearities. The criteria for effectiveall-optical switching is FOM>1. FIG. 9 shows how the FOM for all-opticalswitching depends on the size of the quantum dots. It can be seen thatthe FOM exceeds 1 for a large size dispersion, e.g., for diametersranging from 3 nm to 6 nm. Similar results can be obtained with theother quantum dots described herein, e.g., silicon quantum dots.Therefore, some embodiments of the present invention avoid the need fora substantially monodispersed size distribution of quantum dots whilesubstantially improving switching characteristics and efficiency overprevious uses of quantum dots as nonlinear materials. The effects ofsize distribution and specifically how the FOM of switching depends onthe quantum dot size has not been previously considered in detail.

Shape and Shape Distribution

Quantum dots can be fabricated in a variety of shapes, including (butnot limited to) spheroids, rods, pyramids, cubes, and other geometricand non-geometric shapes. For shapes that are not spherically symmetric,a distribution of orientations can result in an effective broadening ofthe size distribution as seen by incident light. To avoid the need fororientation of quantum dots within a matrix material, the preferredquantum dot shape is spherical, according to some embodiments of theinvention. Spherical quantum dots are also preferred for nanocompositescomprising oriented quantum dots. Alternatively, another preferredembodiment comprises spheroid or substantially spherical quantum dots,with an aspect ratio restricted to between 1±(% size distribution) orwith an aspect ratio between approximately 0.8 and 1.2. In this case,orientation plays an insignificant role in the inhomogeneous broadeningof the spectral features. For similar reasons, the preferred quantum dotwill also be substantially monodisperse in shape. These considerationsregarding the importance of shape and/or shape-distribution constitutean improvement in the use of quantum dots as a nonlinear material.

It should be recognized that an arbitrary shape may still be preferredas long as the relative orientation dependence of the broadening of thelinear and nonlinear optical properties is less than the broadeningresulting from the size distribution of the quantum dot sample.

Crystal Structure of the Core

For reasons similar to those described above for shape, preferredquantum dots according to some embodiments of the invention will have acore with a crystal structure that is spherically symmetric, morepreferably a cubic or diamond crystal structure. Alternatively, thecrystal structure may be non-spherically symmetric, preferablycylindrically symmetric, more preferably a wurtzite crystal structure.

It should be recognized that an arbitrary crystal structure may still bepreferred as long as the relative orientation dependence of thebroadening of the linear and nonlinear optical properties is less thanthe broadening resulting from the size distribution of the quantum dotsample. Once again, the considerations described here regarding theimportance of crystal structure constitute an improvement in the use ofquantum dots as a nonlinear material.

Semiconductor Materials

There are a variety of preferred quantum dot materials for someembodiments of the current invention. For any given application, thepreferred materials can be determined based on the specific opticalrequirements for that application. Examples of such preferred materialsinclude but are not limited to inorganic crystals of Group IVsemiconductor materials including but not limited to Si, Ge, and C;Group II-VI semiconductor materials including but not limited to ZnS,ZnSe, ZnTe, ZnO, CdS, CdSe, CdTe, CdO, HgS, HgSe, HgTe, HgO, MgS, MgSe,MgTe, MgO, CaS, CaSe, CaTe, CaO, SrS, SrSe, SrTe, SrO, BaS, BaSe, BaTeand BaO; Group III-V semiconductor materials including but not limitedto AlN, AlP, AlAs, AlSb, GaN, GaP, GaAs, GaSb, InN, InP, InAs and InSb;Group IV-VI semiconductor materials including but not limited to PbS,PbSe, PbTe, and PbO; mixtures thereof; and tertiary or alloyed compoundsof any combination between or within these groups, including but notlimited to GeSe, SnS, SnSe, PbS, PbSe, PbTe ZnGeAs₂, ZnSnP₂, ZnSnAs₂,CdSiAs₂, CdGeP₂, CdGaAs₂, CdSnP₂, and CdSnAs₂.

Quantum dots of many semiconductor materials can be fabricated, at leastin part, using a variety of methods. Some preferred synthetic methodsinclude those described for Group III-V and Group II-VI semiconductorsas described in U.S. Pat. No. 5,990,479 to Weiss et al., entitled“Organo Luminescent semiconductor nanocrystal probes for biologicalapplications and process for making and using such probes” and issued onNov. 23, 1999; U.S. Pat. No. 5,262,357 to Alivisatos et al., entitled“Low temperature thin films formed from nanocrystal precursors” andissued on Nov. 16, 1993; U.S. Pat. No. 5,505,928 to Alivisatos et al.,entitled “Preparation of III-V semiconductor nanocrystals” and issued onApr. 9, 1996; C. B. Murray et al., “Synthesis and characterization ofnearly monodisperse CdE (E=sulfur, selenium, tellurium) semiconductornanocrystallites, ”J. Am. Chem. Soc. 115, 8706 (1993); and in the thesisof C. Murray, “Synthesis and Characterization of II-VI Quantum Dots andTheir Assembly into 3-D Quantum Dot Superlattices” (MassachusettsInstitute of Technology, Cambridge, Mass. 1995), the disclosures ofwhich are hereby incorporated in their entireties by reference.

The fabrication of some types of shells on quantum dots can be performedusing a variety of methods. Preferred methods include those described inX. Peng et al., “Epitaxial Growth of Highly Luminescent CdSe/CdSCore/Shell Nanocrystals with Photostability and ElectronicAccessibility,” J Am. Chem. Soc. 119, 7019 (1997) and B. O. Dabbousi etal., “(CdSe)ZnS Core-Shell Quantum Dots: Synthesis and Characterizationof a Size Series of Highly Luminescent Nanocrystallites,” J. Phys. Chem.B 101, 9463 (1997), the disclosures of which are hereby incorporated byreference in their entirety.

Two preferred materials for use in quantum dots are silicon andgermanium, according to some embodiments of the invention. Both Si andGe have bulk energy gaps that are less than 1.6 eV, making them idealmaterials from which to fabricate quantum dots that exploit quantumconfinement to enhance optical nonlinearities at telecommunicationswavelengths (where photon energies are typically ˜0.8 eV). The idealchemistry of Group IV materials (as discussed below) further solidifiesthese choices.

In addition, the electron affinity or ionization potential of Group IVmaterials (e.g., Si and Ge) makes them amenable to forming strong andstable covalent bonds with organic and inorganic surface ligands, makingthem ideal for this purpose and for enabling quantum dots that arestable in ambient as well as reasonably extreme environmentalconditions. The significance of this capability can be betterappreciated by recognizing that the surfaces of quantum dots comprisedof more ionic materials often require surfactants or ionic species tocap, which involves less preferable and weaker van der Waals bonds,hydrogen bonds, or ionic bonds. Examples of these more ionic quantum dotmaterials include Group II-VI materials such as CdSe. These more ionicquantum dots often require complex processing to modify the ionicquantum dots so as to enable the more desirable covalent bonding betweenthe quantum dot surface and surface ligands, e.g., a surface layer orlayers comprised of a material different than the core quantum dotmaterial typically needs to be added to the ionic quantum dot surface,wherein the attached surface layer or layers are amenable to covalentbonding to surface ligands. An example of such a surface layer is onecomprised of CdS.

In addition, the chemical properties of Group IV materials (e.g., Si andGe) are such that a stable oxide can be formed that serves to confinecarriers and to passivate the surface to mitigate surface traps.

In addition, the Bohr exciton is relatively large in Ge (˜12 nm), thusproviding a large size range over which the beneficial effect of quantumconfinement, as discussed in various sections herein, are relevant.

A Novel Quantum Dot Material

In one embodiment, the quantum dots are silicon quantum dots orgermanium quantum dots that are surface passivated (or terminated) withan inorganic layer (such as oxides of silicon and germanium) and/ororganic and/or inorganic surface ligands, herein sometimes referred toas SiQDs and GeQDs, respectively. SiQDs and GeQDs as described hereinare novel types of quantum dots that show definitive quantum confinementeffects as manifested by size dependent properties such assize-dependent energy gaps that can be tuned over a very broad range andin particular from the near infrared to the near ultraviolet. Inaddition, SiQDs and GeQDs are stable under a variety of environmentalconditions including ambient (e.g., pressure: ˜1 atmosphere; Gases: ˜70%nitrogen, ˜30% oxygen; Temperature: ˜20-25 ° C.) for desired periods oftime depending on the specific application. A SiQD and a GeQD can becomprised of a substantially Si core for a SiQD and a substantially Gecore for a GeQD. In addition, the “surface” of the SiQD can be comprisedof Si and inorganic elements such as oxygen and/or organic ligands (R).In addition, the “surface” of the GeQD can be comprised of Ge andinorganic elements such as oxygen and/or organic ligands (R).

In one embodiment of the invention, a SiQD comprises a substantiallydefect free silicon crystal core of diameter between approximately 1 nmand 100 nm, preferably between approximately 1 nm and 20 nm, morepreferably between approximately 1 nm and 10 nm, while a GeQD comprisesa substantially defect free germanium crystal core of diameter betweenapproximately 1 nm and 100 nm, preferably between approximately 1 and 50nm, more preferably between approximately 1 and 20 nm. In the case of aninorganic shell surrounding the silicon or germanium core, this shelltypically has a thickness of between approximately 0.1 and 5 nm. Onepreferred inorganic shell is SiO_(n) for SiQD and GeO_(n) for GeQD withn ranging between approximately 0 and 2, preferably ranging betweenapproximately 1.5 and 2, most preferably ranging between approximately1.8 to 2. The chemical composition of the shell (e.g., relative amountsof Si (or Ge) and O) is potentially varying continuously through aportion of the shell and optionally varying discontinuously through aportion of the shell, in which case n can represent an averaged valuewithin the shell. In the case of organic surface ligands terminating thesurface, the SiQD and GeQD can comprise ligand layers comprising organicmolecules with a structure R. R can be any one of a variety ofhydrophobic, hydrophilic, or amphiphilic molecules (a list of preferredsurface ligands is included below). The surface ligands can provide asurface coverage of available silicon (or germanium) and oxygen bindingsites at the surface to provide between approximately 0% and 100%surface coverage, preferably between approximately 20% and 100% surfacecoverage, more preferably between approximately 50% and 100% surfacecoverage, more preferably between approximately 80% and 100% surfacecoverage, with a maximum of one or more complete layers of surfaceligands. R can optionally comprise a plurality of different organicmolecules at a plurality of absolute and relative densities. Finally, aSiQD or GeQD may optionally comprise additional R-groups that do notinteract directly with the quantum dot surface, but rather indirectlythrough other R-groups interacting directly with the surface. In thiscase, surface coverage greater than 100% is possible.

It has long been considered that the production of an ambient-stablesilicon quantum dot or germanium quantum dot with a defined oxide shellcould not be achieved due to difficulties in growing a stable andtrap-free surface oxide shell Thus, the SiQD and GeQD described hereinrepresent a substantial advance.

Methods for fabricating SiQDs and GeQDs in accordance with someembodiments of the invention are discussed below. It should be noted,however, that the current invention refers to SiQDs or GeQDs synthesizedby a variety of other methods in addition to those described herein.Some embodiments of the invention encompass various possible variationsof composition of SiQD and GeQDs that could be made while retaining thegeneral characteristics of a substantially crystalline Si or Ge core anda substantially noncrystalline inorganic (e.g., oxide) shell or organicligand layer.

Method One—“Top Down” Approach

A general method for the formation of quantum dots of some embodimentsof this invention involves a “top down” approach in which “bulk”material is converted to nanostructured material in the form of quantumdots. In this approach, a form of energy is applied to a form of amaterial from which the quantum dot is to be made. The material can bein bulk form, hence the term “top down.” The material in the bulk formis preferably converted to a fine powder, preferably as fine aspossible, more preferably particles in the nanometer size regime (e.g.,between approximately 1 nm and 100 nm) comprised of the material fromwhich the quantum dot is to be made. One advantage of using a finepowder is that it leads to shorter processing times to achieve thedesired quantum dot. The applied energy can be in the form of, forexample, acoustic or vibrational energy (e.g;, sound energy), opticalenergy (e.g., light energy), electrical energy, magnetic energy, thermalenergy, chemical energy, or any combination thereof. More precise sizecontrol of the final quantum dots can be achieved by applying more thanone source of energy to the starting material. Multiple sources ofenergy can be applied simultaneously or they can be applied in varioussequential combinations. This also leads to shorter processing times toachieve the desired quantum dot. It is believed that the applied energyfractures or breaks the starting material into smaller particles and/or“grows” the starting material into larger particles by, for example,fracturing or breaking (e.g., “consuming”) the smaller particles. Inessence, this method evolves the starting material into the desirednanostructured form. This results in the unique quantum dots describedherein in which a stable, well-formed, substantially defect-freeinorganic shell is formed on the surface of the QD, as well as adefect-free interfacial region between the core and the shell. Thisshell imparts a high stability to the QDs under a variety ofenvironmental conditions including ambient. In the specific case of anoxide shell, the oxide is stable and substantially defect-free.

Specific examples of “top-down” methods of formation of quantum dots ofsome embodiments of this invention follow:

EXAMPLE 1 Oxide-terminated SiQDs

A powdered form of Si, from which Si quantum dots can made for someembodiments of this invention, is derived from porous silicon (PSi). Ananostructured PSi layer is removed and made into a fine powder. Energyis then applied in the form of sound energy through sonication and lightenergy through irradiation with a light source. The size of the SiQDs isdetermined by the duration and power of the sonication (with longer andhigher power sonication giving rise to smaller quantum dots) and by thecharacteristics of the light source (with shorter wavelengths and longerirradiation times giving rise to smaller quantum dots).

The method in this example uses sonication periods sufficient to formstable and well-formed quantum dots with stable oxide surfacetermination that were not previously available. In addition, the methodallows the size of the quantum dots to be controlled by the sonicationperiod. In addition, the method also uses light irradiation as a meansto control the size and the size distribution of the quantum dots, whereshorter wavelengths of irradiation and longer irradiation times givesmaller quantum dots and narrower size distributions. This lightirradiation allows better control over the size and range of sizes ofthe quantum dots than previously available, with sizes ranging from ˜1nm to ˜6 nm in diameter in one embodiment of the invention.

The result from the method of this invention is oxide-terminated SiQDsthat are stable under a variety of environmental conditions includingambient. This stability results largely from the stable andsubstantially defect-free oxide shell and interfacial region between thecore and shell.

PSi is formed using a variety of methods that include, but are notlimited to, anodic electrochemical etching of p-doped or n-doped siliconas, for example, described in A. G. Cullis et al., “The structural andluminescence properties of porous silicon,” J. Appl. Phys. 82, 909(1997), the disclosure of which is incorporated herein by reference inits entirety. One preferred method includes starting with p-type (e.g.,Boron-doped) silicon (Si) wafers comprising a plurality of orientations,with the (100) orientation being preferred. The wafer resistivitypreferably ranges from 0.02 Ω-cm to 30 Ω-cm. The wafer is preferablybetween approximately 500-600 microns thick. Electrical contact to thewafer is made through a thin layer of metal (e.g., aluminum or platinum;preferably between approximately 100-500 microns thick) deposited on thebackside of the wafer. Anodic electrochemical etching is performed onthe wafer, which is placed in a solution comprising aqueous hydrofluoricacid (HF, preferably 48 wt %) and ethanol. The weight percentage ofethanol to aqueous HF ranges between approximately 0% and 60%,preferably between approximately 45% and 55%. Various conductingmaterials can be used as the counter electrode in which metals are anexample. Examples of such metals include, but are not limited to,aluminum, copper, brass, and platinum.

The metal layer making electrical contact with the silicon wafer mayoptionally be protected from erosion in the acidic solution by isolatingthe metal layer from the solution. This can be achieved by sealing thesilicon surface with a gasket such that the etching solution issubstantially only in contact with the silicon side of the substrate.Alternatively, the electrode metal can be selected to be relativelyinert under the selected etching conditions or can be selected with athickness great enough to withstand the etching procedures describedbelow.

Electrochemical etching of the Si wafer is carried out for various timedurations (which can range from between approximately 2 and 200 minutes,depending on the starting parameters) using a constant current densityranging from between approximately 5 and 1000 mA/cm² with approximately60 mA/cm² as a preferred current density and approximately 30 minutes asa preferred etching time. After etching, the surface of the Si wafer isleft with a thin layer (between approximately 10 microns and 1 mm inthickness) of nanostructured material, which comprises PSi. The peak ofthe luminescence of the PSi ranges typically from 600 nm to 800 nm (orto greater than 800 nm).

The PSi is optionally rinsed with deionized water, dried under a streamof nitrogen gas, and placed in a vacuum chamber. The chamber isevacuated to a moderate pressure for several hours, preferably less than1 Torr, more preferably less than 500 mTorr, and most preferably lessthan 100 mTorr. The samples are then transferred to a solvent-freeenvironment (e.g., a drybox). The nanostructured PSi layer is thenmechanically removed or scraped (which can be accomplished, for example,with a knife edge or scalpel) from the Si substrate, and the removedmaterial is collected. The nanostructured PSi layer can also beseparated from the Si wafer through a second electrochemical etchingprocess in which low concentration HF/H₂O (preferably between 0.5 and2%) and a high current density (preferably greater than 160 mA/cm²) isused for a few minutes to separate the anodized and nanostructured PSilayer from the Si substrate.

After the PSi layer has been separated from the silicon substrate, thePSi is ground into a fine powder (using, for example, a mortar andpestle and/or a mechanical agitator) yielding about 25 to 40 mg ofpowdered PSi from a wafer surface area measuring approximately 1 inch indiameter. The peak of the PL spectra of the powder is in the redspectral region and ranges any where from 600 nm to greater than 800 nm,depending on the conditions of the electrochemistry. A solvent is thenadded to the powdered PSi. Preferred solvents include, but are notlimited to, acetonitrile, toluene, hexane, methanol, ethanol, ethyleneglycol, and water. In the case of organic solvents, the solvent may bedried over a dehydrant (e.g., calcium hydride or magnesium sulfate),distilled, and degassed prior to being added to the PSi.

The resulting mixture of PSi powder and solvent is placed in a bath andsonicated with acoustic waves or sound energy for a period of time.Although acoustic energy is being disclosed, it is to be understood thatother types of energy may be used, as discussed above. The sonicationcan be accomplished with a variety of equipment that emits acousticwaves or vigorously agitates or shakes the powder, with an ultrasonicbath being a particularly convenient method.

The size and size distribution of the quantum dots in the mixture can becontrolled by varying the duration of sonication. The precise period oftime required for sonication depends on a number of factors that includethe acoustic power of the sonicator, the solvent used, the initial sizeand size distribution of the nanostructures in the PSi powder, etc., andthe characteristics of the sonication should be calibrated for thespecific processing conditions used. A factor determining the optimumtime duration of the sonication is the time required to achieve thedesired size of the resultant quantum dots, i.e., the sonication iscontinued until the desired quantum dot size is reached (e.g., untilcores are formed with diameters within a predetermined or desiredrange). Generally, the size of the quantum dots decreases as thesonication time increases, and the size of the quantum dots can bedetermined by the energy gap or the peak wavelength of thephotoluminescence of the colloidal suspension of quantum dots. Therelationship between quantum dot size and energy gap and therelationship between the PL peak wavelength and quantum dot size forsilicon quantum dots were previously described with respect to FIGS. 2and 3. Therefore, the photoluminescence spectrum of the colloidalsuspension can be periodically taken during the sonication process tomonitor the progress of the sonication. Typically, the PL peakwavelength shifts towards shorter wavelengths (corresponding to a shifttowards smaller peak sizes) during the sonication process. Once thesonication time is calibrated for the processing conditions used to givethe desired quantum dot size, this method can give very consistentresults.

Another factor that can be used to determine the optimum time durationof the sonication is the time required to achieve the desired shells forthe resultant quantum dots, e.g., the sonication is continued untiloxide shells are formed having desired properties as discussed herein.If desired, the sonication time can be calibrated for the processingconditions used to give the desired photoluminescence quantumefficiencies.

As mentioned above, the precise relationship between sonication time andthe quantum dot size that results depends on several parameters that mayneed to be calibrated with each specific fabrication setup andconditions. The following is an example that serves as a point ofreference. With a sonication power of 80 W and with methanol as thesolvent, a sonication period of 10 days resulted in oxide-terminated Siquantum dots with an average size of ˜1.5-1.7 nm in diameter andemitting in the near ultraviolet-blue; a sonication period of 3 daysresulted in quantum dots with an average size of ˜2.5 nm in diameter andemitting in the green; a sonication period of 1 day resulted in quantumdots with an average size of ˜3.6 nm in diameter and emitting in thered.

Upon removal from the ultrasonic bath, the mixture is allowed to settleand is centrifuged, and the supernatant is filtered to remove any largeparticles. Preferred pore sizes of the filter range betweenapproximately 20 nm and 450 nm. Filters can also be used to separate thedifferent sizes of quantum dots. Additionally, other separationtechniques such as chromatography, more specifically gel permeationchromatography or size exclusion chromatography, can be used to separatethe different sizes of quantum dots. The result is a colloidalsuspension of oxide-terminated Si quantum dots (SiQDs) of various sizesthat are stable in a variety of environmental conditions that includeambient (room temperature, pressure, and atmosphere).

As mentioned above, more precise size and size distribution control ofthe final quantum dots can be achieved by applying more than one sourceof energy to the starting material. In one preferred method, two sourcesof energy are applied to the starting material. One additional preferredsource of energy is light energy. In this example of a preferred method,the sample is irradiated with light during or sequentially withsonication. The light source can be a lamp (e.g., Tungsten, Xenon, orMercury), a light emitting diode (LED), a laser, or any other lightsource capable of emitting light at the appropriate wavelengths, where“appropriate wavelengths” is described below. Alternatively, irradiationcan be implemented during the electrochemical etching process (in whichthe etched surface of the Si wafer is irradiated). The size of thequantum dots that result is determined by a number of parametersincluding wavelength, intensity, spectral bandwidth, and duration ofirradiation. Preferably, the wavelength of irradiation should be withinthe spectral region where the light is absorbed by at least a subset ofthe quantum dot sizes to be controlled. Specifically, within a sizedistribution of quantum dots, the longer the wavelength of theirradiation, the larger the size of the resulting quantum dots. Morespecifically, to achieve a specific size SiQD, the sample should beirradiated with photons of energy approximately equal to the energy gapof the desired SiQD. This effect can be accentuated by increasing theduration and/or intensity of irradiation. In particular, the size andsize distribution of the quantum dots in the mixture can be controlledby varying the duration of irradiation. The optimum time duration of theirradiation is the time required to achieve the desired size and/or sizedistribution of the resultant quantum dots, i.e., the irradiation iscontinued until the desired quantum dot size is reached (e.g., untilcores are formed with diameters within a predetermined or desired range)and/or until the desired size distribution is reached (e.g., untilsubstantially monodisperse quantum dots are formed).

For any specific set of synthesis parameters, the precise relationshipbetween irradiation wavelength, irradiation intensity, irradiationduration, and quantum dot size should be calibrated as is done in thecase of the sonication method alone. This can be achieved by monitoringthe energy gap or peak wavelength or spread of the photoluminescence atvarious times during the irradiation as an indicator of the progresstoward the desired quantum dot. Typically, the photoluminescence has apeak wavelength that shifts towards shorter wavelengths (correspondingto a shift towards smaller peak sizes) and a wavelength spread thatnarrows (corresponding to a shift towards narrower spread in sizes)during the irradiation process.

The following serves as examples or points of reference. Simultaneouslyirradiating and sonicating the sample as described above for 5 days with50 mW of laser light at 400 nm results in oxide-terminated SiQDs thatluminesce in the near ultraviolet-blue spectral region; simultaneouslyirradiating and sonicating the sample as described above for 2 days with100 mW of laser light at 532 nm results in oxide-terminated SiQDs thatluminesce in the green spectral region; simultaneously irradiating andsonicating the sample as described above for 0.5 days with 150 mW oflaser light at 620 nm results in oxide-terminated SiQDs that luminescein the red spectral region.

The result of this “top down” approach is oxide-terminated Si quantumdots that are stable in a variety of environmental conditions, includingambient. This capability was previously thought to not be possible. Thisis achieved in the “top down” approach through the establishment of astable and substantially defect-free silicon oxide shell surrounding theSi quantum dot core.

The defect-free nature of the resulting SiQDs is manifested in thequantum efficiency of the photoluminescence from these SiQDs. Thepresence of defects in quantum dots can trap excited carriers (electronsand holes). These trapped carriers can either nonradiatively relax, orthey can radiatively recombine in a defect. Both processes lead to a lowquantum efficiency for the photoluminescence from the quantum dots.Previous quantum dots formed of Si or Ge typically exhibitedphotoluminescence quantum efficiencies of ˜1-5%. In contrast, thephotoluminescence quantum efficiency of the SiQDs made with the methodsof some embodiments of this invention is greater than 6%, preferably atleast or greater than 10%, more preferably at least 20%, more preferablyat least 30%, more preferably at least 40%, and more preferably at least50% (e.g., as high as between approximately 50% and 60%). Thisrepresents the largest photoluminescence quantum efficiency observed forsuch quantum dots.

FIG. 3 shows global PL spectra of six samples of different sizedoxide-terminated Si quantum dots made with the method described herein.FIG. 3 shows that the light emission can be readily tuned from the redto the ultraviolet, and as a result, the size can be readily tuned aswell. And, this light emission is stable in a variety of environmentalconditions including ambient.

The electronic and optical properties of these SiQDs that are made inthis fashion are unique in that they show size dependent properties thatare uniquely consistent with quantum confinement. The optical andelectronic properties of these Si quantum dots are uniquely consistentwith theoretical calculations more sophisticated than Effective Massapproaches such as the Empirical Pseudopotential Method and the TightBinding Method. A comparison of the size dependent energy gap calculatedby these methods and with measurements taken on the SiQDs synthesized bythe method disclosed herein is shown in FIG. 2. The agreement isextremely good and is the best observed for any quantum dots formed ofSi.

The use of light to control the physical size or size distribution ofthe quantum dots in this synthetic process is a particularly novelaspect of certain embodiments of the present invention. Previous methodshave typically used optical excitation not for control of the physicalparameters of the quantum dots but to initiate the chemical reactionneeded for quantum dot formation, i.e., to photolyze the chemicalprecursors. As described above, embodiments of the invention utilizeoptical control over the physical parameters of quantum dots in asynthetic method. This aspect is also applicable to other quantum dotsynthetic procedures and is not limited to those described herein withrespect to SiQDs and GeQDs.

Overall, the size and size distribution of the resulting SiQDs can beprecisely controlled by varying the duration of sonication processing,the strength or intensity of the acoustic energy of the sonicationdevice, the photon wavelength (photon energy) of irradiation, theintensity of irradiation, the spectral width of irradiation, theduration of irradiation, the size and size distribution of the startingmaterial, and the solvent into which the starting material isincorporated. For certain embodiments of the invention, the average SiQDsize can be varied from ˜1 nm to greater than 6 nm with this technique.These average sizes give rise to light emission from the infrared to theultraviolet.

Alternatively, PSi can be fabricated using n-doped Si wafers. In thiscase, a process similar to that described above can be followed.However, the electrochemical etching process may be performed in thedark, and the wafer desirably should be illuminated with a light source(UV light being preferred) during etching in order to generate “holes”(as opposed to electrons) needed in the etching process.

EXAMPLE 2 Oxide-terminated GeQDs

In the synthesis above, the reactants and starting materials can bereplaced with their germanium counterparts for the formation of GeQDs.As shown in FIGS. 4(a) and 4(b), GeQDs of sizes ranging from 1 nm to 16nm have been synthesized using the method according to an embodiment ofthe invention.

The electronic and optical properties of these GeQDs that are made inthis fashion are unique in that they show size dependent properties thatare uniquely consistent with quantum confinement. The optical andelectronic properties of these GeQDs are uniquely consistent withtheoretical calculations more sophisticated than Effective Massapproaches, such as the Empirical Pseudopotential Method and the TightBinding Method. A comparison of the size dependent energy gap calculatedby these methods with measurements taken on the GeQDs synthesized by themethod disclosed herein is shown in FIG. 4(a). The agreement isextremely good and is the best observed for any quantum dots formed ofGe.

Method Two—“Bottoms Up” Approach

In another embodiment, quantum dots can be fabricated from chemicalprecursors. This is essentially a “bottoms up” approach in which thequantum dots can be assembled “atom-by-atom” through chemical synthesis.

The present invention provides general high yield methods ofsynthesizing surface-functionalized quantum dots and, in particular,methods of synthesizing soluble quantum dots of a Group IV semiconductormaterial in a solution at relatively low temperatures.

The methods can be described by reference to the following:

Method 2-a:YX _(a)+Reducing Agent→(Y)X  (1)(Y)X+Capping Agent(R)→(Y)R  (2)Method 2-b:YX _(a) +R _(b) YX _(c)+Reducing Agent→R(Y)X  (3)R(Y)X+Capping Agent(R′)→R(Y)R′  (4)wherein YX_(a) is a source of Y, with Y being Si or Ge, and X isselected from the group consisting of —F, —Cl, —Br, —I, —O—CO—R⁽¹⁾,—NR⁽²⁾R^((3), —O—R) ⁽⁴⁾, —S—R⁽⁵⁾, and so forth, with R⁽¹⁾, R⁽²⁾, R⁽³⁾,R⁽⁴⁾, and R⁽⁵⁾ independently selected from the group consisting ofalkyls, alkenyls, alkynyls, aryls, and so forth. The reducing agent isselected from either activated metals (e.g., Group IA, Group IIA,transition metals, and lanthanides) or hydrides (Group IIIB hydrides,Group IVB hydrides, and transition metal hydrides). Electrochemicalreduction can also be used for reduction. The capping agent(R) and thecapping agent(R′) are sources of surface ligands R and R′, respectively,and can be selected from organometallic reagents, e.g., RM (or R′M),with R (or R′) being a surface ligand (e.g., a linear or branchedalkyls, alkenyls, alkynyls, ether, ester, acid, amide or nitrile moietyhaving between 1 and about 20 carbon atoms). It should be recognizedthat the surface ligands R and R′ can be the same or different. Thecapping agents can also be an alcohol, amine, thiol, and so forth. M ispreferably from Group IA, Group IIA, or Group IIB. In the above, “a”represents an oxidation state or coordination number of Y in the sourceof Y, which is typically 2, 4, or 6, and “b” and “c” are integers thatcan each range from 1 to 6. “a” is typically equal to the sum of “b” and“c”. In method 2-a, (Y)X represent intermediate particles comprisingcores including Y and with surfaces terminated with X, and (Y)Rrepresent quantum dots that are formed with surfaces terminated with R.In method 2-b, R(Y)X represent intermediate particles comprising coresincluding Y and with surfaces terminated with R and X, and R(Y)R′represent quantum dots that are formed with surfaces terminated with Rand R′.

The basic strategy involves solution phase reduction of Si^(a+) orGe^(a+), where a represents the oxidation state of Si or Ge, andsubsequent termination with organic or organometallic reagents. Themethods according to some embodiments of the invention allow mildsynthesis, precise manipulation, functionalization, and interconnectionof the Group IV quantum dots to an extent not previously achieved. Thekey differentiations between previously used methods and the methodsaccording to some embodiments of the invention include one or more ofthe following:

Some embodiments can avoid arduous procedures typically associated withthe use of highly pyrophoric and air-sensitive starting materials, suchas Group IV Zintl compounds or sodium metal. The Group IV Zintl saltsare typically prepared by combining starting materials (e.g., K and Si)at elevated temperature (500-900° C.) in a sealed tube for a few days.As an example, a method of some embodiments of this invention usesmilder and air-stable reducing agents such as magnesium (Mg), otherGroup IIA metals, transition metals, or lanthanides. This makes themethod more amenable to scale up and large scale manufacture.

Some embodiments provide a method in which the reaction conditions areless extreme than required by previous methods. In particular, a methodof some embodiments of the invention avoids the high pressure and hightemperature conditions as sometimes previously used that can producelarge amounts of undesirable insoluble materials.

Some embodiments need not utiltize high energy sonochemical techniquesfor reduction of Si⁴⁺, which has typically produced either smallamorphous particles with ill-defined surface composition or largerinsoluble aggregates with an irregular network.

Some embodiments need not utilize highly toxic gaseous Group IV hydridesand pyrophoric metal hydrides.

The yields from the method of some embodiments in this invention aresignificantly higher than in previously reported methods. In someembodiments, the yields that can be obtained are between approximately35% and 95%.

The size control that can be achieved is greater than previous methods.

The range of sizes possible that can be produced is greater thanachievable with previously reported methods. Some embodiments allowproduction of different sizes of quantum dots that can give rise toinfrared to ultraviolet light emission (e.g., not limited to productionof smaller quantum dots that emit primarily in the blue and blue-greenregion).

The resultant quantum dots are not limited to certain size distributions(e.g., the size distribution control that can be achieved is greater).

Some embodiments afford quantum dots with defined surface compositionand high surface coverage with surface ligands.

The resultant quantum dots are more stable than those produced fromother methods and have the unique properties as described herein.

The resultant quantum dots are more crystalline than those produced fromother methods.

Quantum dots can be produced with higher amounts than achievable withother methods. (e.g., in quantities of at least ten grams).

The functionalization of quantum dots using methods 2-a or 2-b allowsfunctional group inter-conversion at the surface of intermediateparticles produced in equations 1 or 3 with appropriate organic reagentsin equations 2 or 4 to form ligands layers. The intermediate particlestypically will comprise cores that include Y. The organicfunctionalization of these quantum dots imparts favorable solubility incommon organic solvents and compatibility in various matrix materialssuch as organic polymers, inorganic polymers, gels, glasses, and thelike.

Method 2-a is based on controlled chemical reduction of readilyavailable molecular Si^(a+) and Ge^(a+) reagents, where a ranges from 2to 6 and typically from 2 to 4, and the quenching of the correspondingintermediate particles (Y)X with different reagents in a reactionmedium. Two suitable families of reducing agents are activated metalsand hydrides. A nonaqueous reaction medium desirably should be used forthe reduction of silicon and germanium reagents because of the largenegative reduction potential and high oxophilicity of Group IVcompounds. The controlled addition of a capping agent(R) such as RM(R=alkyl, aryl, etc. and M=Li, Na, MgA, ZnA, with A being a halogen, andso forth) to the corresponding intermediate particles at relatively lowtemperatures produces the functionalized quantum dots in high yield.

In a further effort to control quantum dot particles and sizedistributions, terminating agents, R_(b)YX_(c), can be employed usingmethod 2-b (equation 3). R group, in this method, serves as theterminating agent for the quantum dots. The ratio of R_(b)YX_(c) to theYX_(a) reagent in equation 3 can be used as a basic measure of thesurface-to-volume ratio of the quantum dots. A mixture of YX_(a) andR_(b)YX_(c), in the presence of a reducing agent, yields thecorresponding intermediate particles, R(Y)X, which can be treated with acapping agent(R′) such as R′M to displace the remaining leaving groups,X, on the surface of the intermediate particles (equation 4). Thismethodology can, in principle, produce highly functionalized quantumdots.

A preferred method of this chemical synthetic method is described asfollows.

A silicon source, e.g., SiCl₄, is reacted with a reducing agent, e.g.,Mg powder, under an inert atmosphere, e.g., argon. These materials areheated together in a liquid-phase reaction medium. The reaction mediumshould desirably be aprotic. It can be a hydrocarbon, or it could bearomatic. It could be a cyclic or acyclic ether, an aromatic ether, or apolyether. It could contain oxygen, nitrogen, sulfur, and/or phosphorous(so long as it is compatible with the other reagents). It can include anorganic solvent with various combinations of more than one hetero-atomor any combination of the solvents discussed previously. Representativesolvents include alkanes such as heptane, decane, and octadecane;aromatics including benzene, tetralin, and naphthalene; andalkylaromatics such as toluene, xylene, and mesitylene; ethers such asdialkylethers, diarylethers, alkylarylethers, and cyclic ethers; andpolyethers like glymes.

In this process, a Group IV source, such as providing Si⁴⁺, Ge⁴⁺, orGe²⁺, especially in the form of halides or with corresponding 1-20carbon organic substituent (RA, R=organic substituent, A=O, S, N, Si,etc.), is reacted with a reducing agent, such as a Group IA compound,Group IIA compound, transition metal, lanthanide, or hydride, in liquidphase reaction medium at an elevated temperature. Representative GroupIV sources include SiF₄, SiCl₄, SiBr₄, SiI₄, GeF₄, GeCl₄, GeBr₄, GeI₄,GeCl₂, GeBr₂, GeI₂, SiR₄, Si(OR)₄, Si(SR)₄, Si(NR⁽¹⁾R⁽²⁾)₄, Si(O₂R)₄,Si(SiR)₄, GeR₄, Ge(OR)₄, Ge(SR)₄, Ge(NR⁽¹⁾R⁽²⁾)₄, Ge(O₂R)₄, Ge(SiR)₄,Ge(NR⁽¹⁾R⁽²⁾)₂ as well as the dimmers and the higher oligomers of theabove reagents (R, R⁽¹⁾, R⁽²⁾=organic substituent). Representativereducing agents include Li, Na, K, Na/K alloy, Rb, Cs, Be, Mg, Ca, Sr,Ba, Sc, Ti, Zr, Mn, Fe, Co, Ni, Pd, Cu, Zn, Ce, Sm, Gd, Eu, LiAlH₄,NaBH₄, Super-hydride, L-Selectride, RSiH₃, R₂SiH₂, R₃SiH (R=organicsubstituent), and the like. Reducing agents can be provided in a varietyof forms (e.g., as a powder, a liquid, a solid, and so forth). Forcertain reducing agents (e.g., a Group IIA compound such as, forexample, Mg), it is desirable to provide such reducing agents in apowdered form to facilitate reaction with the Group IV source.Alternatively, or in conjunction, it is desirable to provide suchreducing agents in other forms such as in the form of chips, a mesh,dendritic pieces, ribbons, rods, turning or activated (e.g., “Riekemagnesium”, etc.).

One or two of each of these two groups of materials are mixed togetherin the reaction medium (e.g., an anhydrous aprotic solvent) for at leastfew minutes. For some embodiments, the reaction between a source of Sior Ge and a reducing agent is performed by maintaining the reactionmedium at a temperature between approximately −78° C. and 300° C.,preferably between approximately 60° C. and 280° C., and at aroundambient pressure (e.g., about 1 atm) for a period of time betweenapproximately 2 and 48 hrs. For some embodiments, the reaction betweenNa and silicon reagents can require an elevated temperature and aprolonged period to complete. The reflux temperature of the reactionmedium can be used. Elevated pressures of up to about 100 atmospherescan be used to obtain higher temperatures. Suitable temperatures rangebetween approximately 25 and 300° C.

In an additional step in the same pot, the intermediate product, whichis chemically labile, can be functionalized with organic substituentswhen treated with an appropriate reactive material (e.g., capping agent,surface ligands, molecular tethers, terminating agents, passivator,etc.). These reagents can be organometallic reagents, RM (R=surfaceligand such as alkyl, aryl, heteroaryl, and so forth and M=Li, Na, MgA,ZnA, with A being a halogen, and so forth), alcohols, amines, amides,thiols, phosphines, oxyphosphines, acids, silanes, germanes, oxides,silanols, and germanols or their corresponding anion salts.Representative ligand sources include organolithium reagents (e.g.,n-butyllithium, sec-butyllithium, tert-butyllithium, n-hexyllithium, andphenyl lithium); Grignard reagents (e.g., octylmagnesium halide,phenylmagnesium halide, and allylmagnesium halide); alcohols (e.g.,ethanol, isopropyl alcohol, and phenol); amines and thiols (e.g.,diethylamine, octylamine, and hexylthiol); and the like.

For some embodiments, the reaction between the intermediate particles ornanocrystallites and the source of surface ligands can require aprolonged period to complete and can require an elevated temperature.Suitable temperatures range from room temperature to about 100° C. Thereaction can be completed in between approximately 2 and 100 hours atambient temperature. Subsequent work-up affords the organicallyfunctionalized quantum dots as a powder. The subsequent work-uppreferably involves the addition under an inert atmosphere (e.g. argon)of acidic water to destroy the unreacted reducing agent or theorganometallic reagent. The product can be extracted with organicsolvents. The solvent can be a hydrocarbon, an aromatic, or a mixedhydrocarbon fraction. It could be an ether or a polyether. It could beester. It could contain nitrogen, sulfur, and/or halides (so long as itis not very soluble in water). Representative solvents include hexanes,decane, toluene, xylene, diethyl ethers, glyme, dichloromethane,chloroform, ethyl acetate, carbon disulfide, and the like. Theextraction process desirably should be repeated several times to improvethe yield.

The product is a quantum dot powder that can be isolated by removing thesolvent. This can be carried out by evaporation, filtration, and thelike.

The synthetic method described above is associated with yields in therange of 35 to 95%, which are significantly higher than previouslyobtainable. A broad range of particle sizes can be achieved, e.g.,between approximately 1-100 nm.

Various factors can affect particle size, including the nature of thereaction medium, the nature of the reducing agent, the nature of thestarting material, the ratio of reagents employed, concentration,temperature, and pressure employed. The reaction medium employed canplay an important role in the physical properties of the quantum dotproduct. More particularly, coordinating solvents or agents such asoxygen or nitrogen or sulfur or phosphorous containing organic compoundstend to yield quantum dots with larger particle size. In particular, thesize and the size distribution of the quantum dots can be controlled byvarying the coordination ability of the solvent or the co-solvent. As tothe effect of temperature, higher reaction temperatures improve thecrystallinity of the quantum dots and aid in the production of biggerquantum dots. Concentration also affects the particle size, with lowerconcentrations tending to produce smaller quantum dots (the largeramount of solvent effectively causes better heat dispersion from thereacting species).

This invention will be further described by the following examples.These examples are not to be construed as limiting the scope of thisinvention, which is defined by the appended claims.

EXAMPLE 1

A 500-ml three-neck round bottom flask equipped with a stirring bar, areflux condenser, and a thermometer was purged with argon and chargedwith 200 ml of the selected solvent (e.g., glymes (n=1 to 5)) and thereducing agent (e.g., magnesium powder, 0.05 to 0.20 mol). Freshlydistilled YX₄ (0.05 to 0.20 mol) was added dropwise, and the resultingbrown-reddish solution was heated to higher temperatures (e.g., betweenapproximately 60 and 280° C.) for a period of time (e.g., betweenapproximately 2 and 100 hrs, typically between approximately 2 and 48hrs). The resulting mixture was cooled to about −20° C. and treated withan excess amount of the capping agent (e.g., 1.8 M solution ofphenyllithium), which was added dropwise to keep the temperature belowroom temperature. After the reaction mixture was stirred at ambienttemperature for a period of time (e.g., between approximately 2 and 48hrs), it was quenched with dilute protic acid (pH˜2) and extracted withan organic solvent (e.g., toluene). The combined organic extracts werewashed with water and dried over a drying agent (e.g., sodium sulfate).The solvents were removed under reduced pressure, and traces of thesolvents were removed by precipitation with a nonsolvent (e.g.,pentane). After centrifugation or filtration, the product was collectedand dried in a vacuum oven. The product can be purified by columnchromatography (e.g., silica, CH₂Cl₂/methanol, 95/5).

EXAMPLE 2

The preparation of Example 1 is repeated using sodium as the reducingagent.

EXAMPLE 3

The preparation of Example 1 is repeated using barium as the reducingagent.

EXAMPLE 4

The preparation of Example 2 is repeated using a mixture of 35%/ 65% (byvolume) diglyme/xylenes as the reaction medium.

EXAMPLE 5

The preparation of Example 1 is repeated using diphenyl ether as thereaction medium.

EXAMPLE 6

The preparation of Example 1 is repeated using tetraglyme as thereaction medium.

EXAMPLE 7

The preparation of Example 1 is repeated using n-butyllithium as thecapping agent.

EXAMPLE 8

The preparation of Example 1 is repeated using 3-butenylmagnesiumbromide as the capping agent.

EXAMPLE 9

The preparation of Example 1 is repeated using allylmagnesium bromide asthe capping agent.

EXAMPLE 10

The preparation of Example 1 is repeated using 4-methoxyphenylithium asthe capping agent.

EXAMPLE 11

The preparation of Example 1 is repeated using pentafluorophenyllithiumas the capping agent.

EXAMPLE 12

The preparation of Example 1 is repeated using perfluorohexyllithium asthe capping agent.

EXAMPLE 13

The preparation of Example 1 is repeated using sodium ethoxide as thecapping agent.

EXAMPLE 14

The preparation of Example 1 is repeated using silicon tetrabromide asthe source of silicon.

EXAMPLE 15

The preparation of Example 1 is repeated in a sealed pressure reactor at260° C.

EXAMPLE 16

The preparation of Example 1 is repeated using a mixture of70%/10%/10%/10% (by molar ratio)germaniumtetrachloride/phenyltrichlorogermane/diphenyldichlorogermane/triphenylgermaniumas germanium source and terminating agents, R_(b)YX_(c).

Preferred Surface Ligands and Molecular Tethers

As described in the previous sections, the ligand layer can serve topassivate the surface of a quantum dot and eliminate surface defects. Italso facilitates compatibility with matrix materials. This is furtherexplained as follows. Fluoropolymers are a group of desirable materialsfor optical applications because of their unique properties.Fluoropolymers, in general, have low indices of refraction (e.g., incomparison with regular hydrocarbon polymers) and thus low intrinsicscattering loss. They also, in general, exhibit low absorption loss asthey are typically comprised of little or no carbon-hydrogen bonds. Theyare hydrophobic and thus low in moisture absorption. They, in general,are chemically and thermally inert and thus compatible in demandingenvironments and extreme process conditions in device fabrication.Because of their inertness, fluoropolymers are nearly non-mixable withmany materials, such as conventional quantum dots. Embodiments of thecurrent invention provide a novel approach to circumvent thecompatibility issue by introducing fluorinated surface ligands to thesurface of quantum dots (e.g., as in Example 11 and 12 of the precedingsection). The quantum dots, which are terminated with a ligand layer offluorinated surface ligands, can now be incorporated into, for example,Cytop® brand polymer (a perfluorinated polymer from Asahi), facilitated,for example, by using a solvent vehicle, CT-SOLV 180 from Asahi.

The following are preferred surface ligands of the ligand layer,according to some embodiments of the invention. This list, which is notintended to be exhaustive, describes a number of surface ligands havingdesirable physical characteristics that can be used to form ligandlayers for SiQDs or GeQDs. In the following, Y is Si or Ge, and Y—C,Y—O, Y—S, Y—Si, and Y—N denote covalent bonds between Si or Ge and a Catom, an O atom, a S atom, a Si atom, and a N atom, respectively. Otherpreferred surface ligands, not listed below, can contain a P or a Seatom that is covalently bonded to Si or Ge.

Y—C

-   -   A) Alkyls        -   a. Simple aliphatic alkyl groups (e.g., methyl, ethyl,            propyl, etc.)        -   b. Branched and cyclic alkyl groups (e.g., iso-propyl,            tert-butyl, cyclohexyl, etc.)        -   c. Substituted alkyl groups (e.g., 4-cyanobutyl,            3-ethoxy-3-oxopropyl, etc.)        -   d. Perfluorinated alkyl groups (e.g., linear, branched, or            cyclic)    -   B) Alkenyls        -   a. Simple isolated double bonds (e.g., 1-hexenyl,            1-dodecenyl, etc.).        -   b. Substituted alkenes (e.g., 6-heptenenitrile, etc.)        -   c. Conjugated polyenes (e.g., pentadienyl etc.)        -   d. Polymerizable alkenes (e.g., allyl, 3-butenyl, 2-butenyl            etc.)    -   C) Alkynyls        -   a. Simple isolated alkynes (e.g., hexynyl, octynyl, etc.)        -   b. Substituted alkynes (e.g., phenylethynyl, etc.)        -   c. Polymerizable alkynyls        -   d. Perfluoro alkynyls    -   D) Aromatics and Aromatic Heterocycles        -   a. Phenyls, Pyridyls, Thienyl, etc.        -   b. Substituted Aromatics and Aromatic Heterocycles            -   i. With electron withdrawing groups (nitro, nitrile,                fluoro, perfluoro, carboxylate, e.g., 4-cyanophenyl,                etc.)            -   ii. With electron donating groups (amino, alkoxy, e.g.,                4-methoxyphenyl, etc.)    -   E) Conjugated Aromatics, Aromatic Heterocycles, and Polyenes        (poly is referred to well defined oligomers)        -   a. Polyenes        -   b. Poly(p-phenylene)        -   c. Poly (diacetylene)        -   d. Poly(triacetylene)        -   e. Poly(p-phenylene vinylene)        -   f. Poly(p-phenylene ethynylene)        -   g. Polythiophene        -   h. Polypyrrol        -   i. Polyaniline        -   j. Poly(phenylene sulfide)    -   F) Cyanide        Y—O    -   A) Hydroxy, Alkoxy, etc.        -   a. Diol, triol, polyol, etc.        -   b. Cholesteryl group        -   c. Trisubstituted siloxy    -   B) Carboxylate    -   C) Phenoxy    -   D) Siloxy    -   E) Cyanate    -   F) Inorganic Oxides        Y—S    -   A) Thioalkyl    -   B) Thioaryl    -   C) Thiocyanate    -   D) Silylthio        Y—Si    -   A) Substituted silyl group    -   B) Tri-substituted silyl group with one or more functional        groups        Y—N    -   A) Amino group (e.g., linear, branched, aromatic, or cyclic)    -   B) Mono and di-substituted amines    -   C) Imino group (e.g., linear, branched, aromatic, or cyclic)    -   D) Silylamino

FIGS. 10(a) and 10(b) show PL spectra of organic-terminated Si quantumdots, and FIGS. 11 (a) and 11(b) show PL spectra of organic-terminatedGe quantum dots. The Si and Ge quantum dots were made with the methodsdescribed herein. The PL spectra show that the light emission can bereadily tuned from the red to the ultraviolet by exciting quantum dotsof different sizes. The PL spectra are obtained by optically excitingthe quantum dots with wavelengths shorter than the wavelength at theabsorption edge of the quantum dots. This light emission is stable inambient conditions. This stability is due in large part to the relativecompleteness and stability of the surface termination, e.g., the surfacetermination and the interface between the core and the surfacetermination is substantially defect free. In FIGS. 10(a) and 10(b), thesurfaces of the Si quantum dots are terminated with 4-methoxyphenylgroups. In FIG. 10(b), the vertical axis represents a normalizedphotoluminescence signal from FIG. 10(a). In FIGS. 11(a) and 11(b), thesurfaces of the Ge quantum dots are terminated with butyl groups. InFIG. 11(b), the vertical axis represents a normalized photoluminescencesignal from FIG. 11(a). Similar results can be seen for Si quantum dotshaving surfaces terminated with ethoxy groups and Ge quantum dots havingsurfaces terminated with methyl groups.

The electronic and optical properties of these organic-terminated SiQDsand GeQDs that are made in this fashion are unique in that they showsize dependent properties that are uniquely consistent with quantumconfinement. The optical and electronic properties of these SiQDs andGeQDs are uniquely consistent with theoretical calculations moresophisticated than Effective Mass approaches, such as the EmpiricalPseudopotential Method and the Tight Binding Method. A comparison of thesize dependent energy gap calculated by these methods with measurementstaken on the SiQDs and GeQDs synthesized by the method disclosed hereinshow that the agreement is extremely good and is the best observed forany quantum dot formed of Si or Ge.

According to some embodiments of the invention, nanocomposite materialscomprising quantum dots that are surface-terminated with various organicgroups and dispersed in processible matrix materials such as organicpolymers or sol-gels can exhibit new quantum phenomena. This new quantumphenomena in turn allow a large variety of new applications (such asall-optical switching) and the fabrication of device structures usinglow cost processing techniques (e.g., spin coating or dipping).Described herein are several novel synthetic schemes to fabricate thesequantum dots and to functionalize their surfaces with molecular speciesthat are chemically bonded to the surface for stability and robustness.Use of such functionalized quantum dots avoids the need for expensiveand specialized fabrication equipment and facilities. The synthesis ofthese nanostructures can be readily implemented in many laboratories.

As discussed above, the value of this quantum dot nanostructure canderive from molecular tethers serving multiple functions. The moleculartethers may be active in a variety of ways, e.g., electrically,chemically, mechanically, or optically active. This enables precisecontrol of the electrical, optical, transport, chemical, and physicalinteractions between quantum dots and the surrounding matrix material orthe properties of individual quantum dots. These molecular tethers canbe a key innovation needed to develop new devices and applications.Examples of particularly preferred embodiments of optically activemolecular tethers are molecules with polarized or polarizable sectionsor with large polarizabilities, donor-acceptor molecules,hetero-molecules, and charge transfer molecules.

Another major innovation comes from collective phenomena resulting fromnanocomposite materials that include coupled quantum systems such ascoupled quantum dots. The ability to attach active molecular tethers tothe quantum dot surface allows coupling quantum dots together in variousone, two, and three-dimensional configurations or arrays to initiatemultiple quantum interference interactions between quantum dots that maybe applied towards novel devices. The length and properties of thesemolecular tethers can be tailored to enhance or generate specificquantum phenomena such as enhanced nonlinear optical properties. Forinstance, molecular tethers can provide charge transport between two ormore interconnected quantum dots. For certain embodiments of the presentinvention, the quantum dots can be massively interconnected to an extentthat is unlike previous efforts. The massively interconnected quantumdot system can be comprised of more than 2 interconnected quantum dots,preferably more than 10 interconnected quantum dots, preferably morethan 1000 interconnected quantum dots, and most preferably more than 10⁹interconnected quantum dots. For certain embodiments of the invention,the massively interconnected quantum dot system can be comprised of twoor more massively interconnected quantum dot subsystems, whichsubsystems may or may not be connected. The quantum dots can beinterconnected via the formation of chemical bonds between appropriatemolecular tethers on different quantum dot surfaces. This, in turn, canbe performed using the functionalization of the quantum dot surfaces asdescribed earlier herein. After the quantum dot surface isfunctionalized, the interconnection can proceed via chemical reactionbetween surface functional groups, e.g., conjugated species, aromatics,etc. As a result of such interconnection, a large variety ofnanostructures is possible:

-   -   (1) n quantum dots coupled in a linear structure or array.    -   (2) n quantum dots coupled in an arbitrary 2-dimensional        structure or array.    -   (3) n quantum dots coupled in an arbitrary 3-dimensional        structure or array (e.g., to produce new lattice structure and        new materials with tailorable properties.).    -   (4) n quantum dots attached to a polymer backbone to give        controllable densities of quantum dots. These quantum dots can        be coupled with other species (e.g., electron donating or        accepting molecules) onto the polymer backbone to generate other        new phenomena and applications.

These nanostructures can have the properties of single quantum dots oran ensemble of quantum dots, which will be determined by the nature ofthe molecular tethers. This approach can be important for exploitingcollective excitations in quantum dot systems towards innovativedevices. These new nanostructures represent an important innovation innanotechnology. Examples of particularly preferred embodiments ofmolecular tethers that can be used to interconnect quantum dots in thisfashion and to generate controllable collective phenomena includeconjugated species such as conjugated polymers (e.g., alkenes, alkynes,and aromatics).

The uniqueness of the synthetic process described above is manifestedin, but is not restricted to, the following properties of the resultantquantum dots and nanocomposite materials formed of such quantum dots:(1) extremely large optical nonlinearities are manifested, e.g., inlarge values of Re[χ⁽³⁾ _(ijkl], with values as high as) 10⁻⁵ cm²/W to10⁻⁴ cm²/W. Previous materials with optical nonlinearities in theinfrared and more specifically in the important telecommunicationsregion of 1500 nm to 1600 nm typically have values of nonresonantdegenerate γ of ˜10⁻¹² cm²/W to 10⁻¹¹ cm²/W or less. As a result, theoptical nonlinearity, e.g. Re[χ⁽³⁾_(ijkl], of the quantum dots of this invention is ˜)10⁶ to 10⁸ timeslarger than such previous materials; (2) stability of the quantum dotsin a variety of environmental conditions including ambient; (3)stability of the infrared to ultraviolet emission in a variety ofenvironmental conditions including ambient; (4) control over the size ofthe quantum dots such that the light emission can be size-tuned from theinfrared to the ultraviolet; (5) control over the size of the quantumdots such that the nonlinear optical properties give largefigures-of-merits that surpass those required for effective all-opticalswitching; (6) the nonlinear optical properties are such thatall-optical switching occurs in a very short time (depending on thenature of the nonlinear optical mechanism (e.g., resonant ornon-resonant, the switching time can range from picoseconds to less than60 femtoseconds); (7) low switching energy (<<1×10⁻¹² Joules); (8)non-degenerate (e.g., control and data beams with different wavelengths)all-optical switching where the wavelengths of the relevant beams can bedetuned from each other over a very broad spectral range (>>100 nm) andstill maintain effective all-optical switching; (9) all-opticalswitching can occur throughout a broad wavelength range (e.g., from 400nm to 1600 nm).

Preferred Matrix Materials

According to some embodiments of the current invention, the matrixmaterial that is used to host quantum dots can be selected from a broadrange of materials due in large part to the versatile surfacetermination of the quantum dots as discussed above. These matrixmaterials can include, for example, organic and inorganic polymers orglasses with different properties including mechanical strength, opticaltransparency, lightwave transmissivity, thermal stability, dimensionalstability, low temperature flexibility, moisture absorption, andchemical inertness.

The matrix materials in some embodiments of the current invention arepreferred to be highly transparent and low absorption in the wavelengthrange from 600 nm to 2 μm. Also, they are preferred to be highlycompatible with quantum dots so that a desired amount of quantum dotscan be readily incorporated into the resulting nanocomposite materialwithout degrading optical and mechanical properties. Polymers withspecial functional groups may be selected to facilitate solubilityinteractions and enhance compatibility with quantum dots. This isfurther explained as follows. Polymers with Lewis acid (base) functionalgroups, for example, can be selected to host quantum dots which aresurface-functionalized with Lewis base (acid) surface ligands. Anotherexample is to take advantage of hydrogen-bonding interactions. Polymersof hydrogen-bonding donors (acceptors) are the preferred matrixmaterials for quantum dots which are surface functionalized withhydrogen-bonding acceptors (donors). Additionally, polymers with strongdipolar groups are the preferred matrix materials for quantum dots whichare engineered with strong dipolar surface ligands. The strongintermolecular interactions described above greatly enhancecompatibility between the quantum dots and the matrix materials.Therefore, high contents of quantum dots can be readily incorporatedinto the matrix materials while maintaining desired uniformity andhomogeneity (e.g., allowing the quantum dots to be substantiallyuniformly dispersed throughout the matrix materials). Additionally,block copolymers can be used to further enhance compatibility byskillful selection of monomer units and block length. As a result, ananocomposite material comprising quantum dots and a matrix material canbe engineered to be of high optical quality and low scattering loss.More importantly, the linear and nonlinear index of refraction can betuned for a variety of applications primarily by adjusting the contentof the quantum dots and by selecting the matrix material, according tosome embodiments of the invention.

In addition to the optical properties and compatibility with the quantumdots, the preferred matrix material desirably should meet otherrequirements for a specific application. Thus, other properties can beconsidered in the selection of matrix materials.

One preferred matrix material is selected from a group of polymers withhigh glass transition temperature, T_(g), such as polyimides,fluoropolymers (e.g., Teflon AF® brand fluoropolymers available fromDuPont), polymers derived from B-staged bisbenzocyclobutene monomers(e.g., Cyclotene® brand resins and Cyclotene® brand fluorinated resinsavailable from The Dow Chemical Company), phenolic resin, andfluorinated poly(aryl ether sulfide), for applications where thermalstability is important.

Another preferred matrix material is selected from a group of polymerswith low T_(g) such as poly(isobutylene), poly(diphenoxyphosphazene),and fluorinated acrylate (ZPU series from Zen Photonics Co., LTD) forapplications where low temperature flexibility and low birefringence aredesired.

Another preferred matrix material can be selected from photosensitivepolymers, such as fluoropolymers (e.g., Cytop® brand fluoropolymersavailable from Asahi), poly(methyl methacrylate), and photoresists tofacilitate lithographical fabrication of devices.

Another preferred matrix material is selected from a group ofcross-linkable polymers for applications where isotropic homogeneity ordimensional stability is required.

Another preferred matrix material is a blend of two or more polymerswhich are engineered to tailor the optical and mechanical properties andthermal and chemical stability.

Another preferred matrix material is a copolymer including random andblock copolymer.

Another preferred matrix material is a homopolymer including, but notlimited to, the following:

-   Poly(vinyl alcohol)-   Poly(vinyl butyral)—other-   Poly(vinylcarbazol)-   Poly(vinyl fluoride)-   Poly methyl vinyl ether-   Polyethylene-   Polypropylene-   Polystyrene-   Poly(vinyl pyridine)-   Polyimides-   Poly(ethylene oxide)-   Photoresist (positive or negative)-   Cyclotene®-   Fluorinated Cyclotene®-   Cytop®-   PMMA-   Fluorinated acrylates-   poly(siloxanes)-   Poly(silanes)-   Poly(diphenoxyphosphazenes)-   Poly(vinyl ferrocene)-   Polycarbonate-   Polystyrene-   Poly(cyclic olefen) such as Zenor® and Zenex®-   Teflon® AF®

Another preferred matrix material is a glass including, but not limitedto, the following:

-   Sol-gel derived glasses-   Organically modified glasses-   Spin-on glasses-   Flow-glass-   Dielectrics such as Low K FlowFill™ brand dielectrics of Trikon    Industries-   Dielectrics such as Black Diamond™ brand dielectrics of Applied    Materials, Inc.    Preferred Methods of Use

The engineered nonlinear nanocomposite materials of some embodiments ofthe present invention can be incorporated into an optical device by avariety of methods, including a variety of standard methods known in theart. The flexibility to process the nanocomposite material ofembodiments of the current invention, desirably independent of thenonlinear optical properties, is a key benefit of embodiments of thecurrent invention. By selecting an appropriate matrix material andsolvent, engineered nonlinear nanocomposites can be deposited usingspin-coating, spin casting, dip coating, spraying, blade application,screen printing, and other methods commonly used in the process ofstandard semiconductor micro-fabrication.

While processes like spin-coating have been used in other contexts, thecombination of tuning the optical and mechanical properties of anengineered nonlinear nanocomposite material followed by spin-coating, orthe like, is unlike previous capabilities. Traditional nonlinearmaterials known in the art have chemical and mechanical properties thatare directly linked to their optical properties. The processingtechniques that can be used to incorporate these materials are thereforeoften limited to those that are compatible with the materialsthemselves. For instance, LiNbO₃ is a crystal and can therefore not beincorporated by spin coating.

The steps of incorporating a nonlinear nanocomposite material into adevice by selecting desired optical properties, substantiallyindependently selecting desired chemical and/or mechanical properties tofacilitate incorporation by a particular technique (e.g., spin-coating),and then incorporating the nanocomposite material using that techniquerepresents a substantial improvement over previous incorporationmethods. While spin-coating has been discussed herein as a specificexample of a standard method of materials incorporation, this is donestrictly for exemplary purposes and should not be considered to limitthe scope of the invention.

For example, desired optical properties such as linear index ofrefraction and γ can be established by selecting or tuning at least oneof a chemical composition of quantum dot cores, a chemical compositionof quantum dot shells, a peak size of quantum dots, a thickness of theshells, a chemical composition of ligand layers, a chemical compositionof a matrix material, a concentration of the quantum dots in the matrixmaterial, and a degree of interconnection of the quantum dots (e.g.,using molecular tethers). Other desired optical properties such assingle-photon and multi-photon absorption characteristics can beestablished by selecting or tuning at least one of a chemicalcomposition of quantum dot cores, a chemical composition of quantum dotshells, a peak size of quantum dots, and a thickness of the shells.Desired chemical and mechanical properties can be established byselecting or tuning at least one of a chemical composition of ligandlayers and a chemical composition of a matrix material. As discussedpreviously, at least two of these desired properties can besubstantially independently established, according to some embodimentsof the invention.

In addition to standard incorporation techniques, other methods ofdeposition such as layer-by-layer growth using polymers with alternatingand complementary functionalities, as pioneered by Gero Decher anddescribed in T. Sasaki et al., “Layer-by-Layer Assembly of TitaniaNanosheet/Polycation Composite Films,” Chem. Mater. 13, 4661 (2001), thedisclosure of which is incorporated herein by reference in its entirety,can be used to create films and coatings of laminated layer structuresin the required thickness with desired density of quantum dots.

All of the same processing techniques are also possible for quantum dotsolids, including the ability to perform layer-by-layer growth. Hereagain, the process of selecting the chemical properties of the surfaceligands and solvent to facilitate incorporation by a particulartechnique, desirably independent of the optical properties, represents asignificant improvement over previous incorporation methods.

The following provides some additional preferred methods ofincorporating an engineered nonlinear nanocomposite material into avariety of devices:

The engineered nonlinear nanocomposite material can be dispersed in apolymer and subsequently dissolved in an appropriate solvent to create afluid of sufficient viscosity to generate the desired thickness of afilm. The film thickness can be easily tailored by varying the solventcontent and therefore the viscosity. The specific quantum dot surfacechemistry is selected for compatibility with the selected polymer andsolvent to be used. Some preferred materials include: Dow Chemical'sCyclotene®, which is B-staged divinylsiloxane-bis-benzocyclobutene withMesitylene and minor portions of other organic compounds; poly (methylmethacrylate) (PMMA); photoresists (both positive and negative) used insemiconductor manufacturing; and so forth.

The engineered nonlinear nanocomposite material is dispersed in asuitable carrier fluid or solvent and applied evenly over the desiredsurface. Heat, vacuum, IR radiation, and/or an inert carrier gas arethen used to remove the carrier fluid, giving rise to a film of theengineered nonlinear nanocomposite material on the device.

The engineered nonlinear nanocomposite material is dispersed in acarrier gas, which is either reactive or inert. Appropriate carriergasses include, but are not limited to, SiH₄, N₂, H₂, O₂, and N₂O. Thegases are allowed to react under appropriate conditions of heat and/orplasma to cause a CVD film to be deposited on a substrate of choice. Inthis embodiment, a preferred substrate is a silicon wafer, optionallycomprising lithographic structures or patterns on the surface.

The engineered nonlinear nanocomposite material is incorporated into asputter target, optionally using procedure (i) above. Alternatively, apure target of a desired matrix material could be used (e.g., organic orinorganic targets, preferably SiO₂), and the engineered nonlinearnanocomposite material is introduced in a gas in a sputter chamber. Theengineered nonlinear nanocomposite material is then incorporateddirectly into a growing sputtered film.

The engineered nonlinear nanocomposite material is heated and caused tovaporize. The material vapors are then transported to a desired surfaceand condensed by keeping the surface at a suitable temperature. Theresult is a solid film deposited on a device.

The same concepts can be used in systems that deposit Low K materialsuch as Low K FlowFill™ brand dielectrics from Trikon Industries orBlack Diamond™ brand dielectrics from Applied Materials, Inc., thusincorporating quantum dots into low k films for even better control ofthe index of refraction and processability.

Preferred Nanocomposite Materials

Embodiments of the current invention comprise a nanocomposite materialwith a controllable set of optical, mechanical, chemical, and electronicproperties. The nanocomposite material can comprise quantum dotsdispersed in an organic and/or inorganic matrix material. The matrixmaterial may be either doped or undoped with molecular species, with adensity of quantum dots therein such that the index of refraction (e.g.,the linear index of refraction or the overall index of refraction) ofthe nanocomposite material falls between approximately 1.3 and 5.0. Someembodiments of the nanocomposite material comprise at least or more than10% by weight of the quantum dots (e.g., at least 20% by weight of thequantum dots, at least 30% by weight of the quantum dots, at least 40%by weight of the quantum dots, or at least 50% by weight of the quantumdots, such as between approximately 50% and 60% by weight of the quantumdots). More particularly, some embodiments of the nanocomposite materialcan comprise the above discussed weight percentages of the quantum dotswith little or no agglomeration or aggregation of the quantum dots andwith the quantum dots substantially uniformly dispersed throughout thematrix material. Also disclosed are nanocomposite materials (e.g.,quantum dot solids) such that the density of quantum dots within thenanocomposite material is between approximately 0.005% and 75% byvolume. Optionally, the index of refraction of the nanocompositematerial can be additionally tuned by selecting a matrix material with aspecific index of refraction and/or further doping the matrix materialto modify that index. This provides additional control over the opticalcharacteristics. Optionally, the matrix material can be a polymerizablematerial with a desired index of refraction. The index of the refractioncan be further fine-tuned by cross-linking via various activationmechanisms including thermal, photo illumination, plasma, and highenergy radiations. The matrix material in which the quantum dots aredispersed may optionally have an intrinsically high χ⁽³⁾. The matrixmaterial may optionally be an intrinsic matrix of a quantum dot solid.

The nanocomposite materials according to some embodiments are preferablyoptically pure, with a homogeneous distribution of quantum dotsdispersed therewithin. These quantum dots may be substantially uniformlydispersed as individual dots or as aggregates of controlled sizes (e.g.,smaller aggregates up to massively interconnected quantum dotsubsystems). The engineered nanocomposite materials are preferablyoptically homogeneous and uniform, so that little or no scatteringand/or mode disruption result from light passing through or past thematerial, as the specific application demands. For certain applications,close-packed micron- or sub-micron-sized clusters of quantum dotsdispersed in a polymer or other matrix material with a filling fractionoptimized to enhance local field effects may be preferable.

In addition, nanocomposite materials of some embodiments preferably havean optical nonlinearity, such as Re[χ⁽³⁾ _(ijkl)] contributing to γ.,e.g., under degenerate conditions, such as nonresonant degenerateconditions in a wavelength range-of-interest of between approximately10⁻¹² and 10⁻⁵ cm²/W, more preferably between 10⁻¹⁰ and 10⁻⁵ cm²/W andmost preferably between 10⁻⁸ and 10⁻⁵ cm²/W. In particular, certainembodiments of the nanocomposite material have γ being at least 10⁻⁹cm²/W (e.g., at least 10⁻⁸ cm²/W or at least 10⁻⁷ cm²/W) when irradiatedwith light having a wavelength between approximately 3×10⁻⁵ cm and2×10⁻⁴ cm. More particularly, certain embodiments of the nanocompositematerial have y being at least 10⁻⁹ cm²/W (e.g., at least 10⁻⁸ cm²/W orat least 10⁻⁷ cm²/W) when irradiated with light having a wavelengthbetween approximately 1.25×10⁻⁴ cm and 1.35×10⁻⁴ cm or betweenapproximately 1.5×10⁻⁴ cm and 1.6×10⁻⁴. For γ under non-degenerateconditions, both relevant wavelengths (e.g., wavelengths correspondingto trigger and data signals) can lie within the wavelength ranges statedabove. Optical characteristics of the disclosed nanocomposite materialscan be evaluated in a variety of configurations and are not restrictedby the specific examples described herein. One of skill in the art willappreciate that the linear and nonlinear optical properties of amaterial can be evaluated using methods such as Z-scan, FWM, cross-phasemodulation, nonlinear phase shift in an interferometer, nonlinearetalons, and so forth.

The mechanical properties of nanocomposite materials are preferablyselected to be compatible with incorporation into devices selected fromthe list of: planar waveguides, nonplanar waveguides, optical fibers,waveguide cores, waveguide claddings, free-space optics, and hybridoptical devices. Such nanocomposite materials can be used in a varietyof optical devices for switching, modulating, and manipulating light inways such as for an optical switch, an optical cross-connect, awavelength converter, and the like, as well as combinations thereof.

The nanocomposite materials described herein can have a number of keyattributes lacking in other materials. For instance, the nanocompositematerials can have an extremely large optical nonlinearity. This opticalnonlinearity can be represented by the real part of various tensorelements of χ⁽³⁾, which include χ⁽³⁾1111, χ⁽³⁾1212, χ⁽³⁾1221, χ⁽³⁾1122,and various permutations of the energies of the optical fields involved,e.g., χ⁽³⁾ _(ijkl) (ω₄,; ω₁, ω₂, ω₃). According to some embodiments ofthe invention, the value of the real part of these tensor elements fallsin the range of 10⁻⁹ cm²/W to 10⁻⁴ cm²/W. As a result, the nanocompositematerials allows all-optical devices to be made that can be effectivelyswitched or controlled with very low intensity light such as light fromcontinuous wave laser diodes and also LEDs in some cases. Thiscapability is highly sought after but has not been previously achievedin a satisfactory manner.

Also, the nanocomposite materials described herein can exceed variousFOM criteria for effective all-optical switching. In particular, certainembodiments of the nanocomposite material has a FOM that is at least 1(e.g., at least 1.5 or 1.8), where this particular figure-of-merit canbe defined as 2γ/βλ, where β is a two-photon absorption coefficient ofthe nanocomposite material expressed in cm/W, and λ is a wavelengthbetween approximately 3×10⁻⁵ cm and 2×10⁻⁴ cm, preferably betweenapproximately 1.25×10⁻⁴ cm and 1.35×10⁻⁴ cm or between approximately1.5×10⁻⁴ cm and 1.6×10⁻⁴. It should be recognized that other definitionsfor the FOM may be used instead. This is particularly differentiatingsince, though other materials may possibly have large nonlinearrefractive indices, the linear or nonlinear losses such as thatoriginating from two photon absorption are often sufficiently large sothat the FOM is inadequate, the thermal properties are sufficiently poorsuch that the FOM is inferior, and the temporal response is considerablyslower than with the nanocomposite materials described herein. Anotherimportant consequence is that significantly shorter lengths of thenanocomposite materials described herein are required for effectiveall-optical switching devices. Thus, significantly smaller and fasterdevices can be made. As an example, rather than requiring centimeters ormore of a conventional material to effectively switch light, devices canbe made with the nanocomposite materials with lengths of approximatelyten microns to a few millimeters.

In addition, the nanocomposite materials described herein are relativelysimple and inexpensive to make, are more easily processed, arecompatible with a large number of other material systems, and can beincorporated more readily into various device structure and in nearlyany device size. Epitaxial growth of the nanocomposite materialsdescribed herein is typically not required, which can be an advantagesince epitaxial growth is typically an expensive process that is oftennot amenable to simple processing or large area devices and is oftenincompatible with many other material systems (since it requiresepitaxial growth on material systems that are lattice-matched toitself). The nanocomposite materials can be deposited on varioussubstrates, substantially independently of their size, surface area, andsurface nature, in the form of films or coatings of varying thicknessesand can be formed into structures of various shapes and sizes.Importantly, these films, coatings, and structures made from thenanocomposite materials can be manufactured with a number of simple andinexpensive fabrication techniques such as spin coating, spray coating,doctor blading, and dip coating at ambient temperature and pressure, orusing conventional molding processes for engineering plastics andelastomers.

And, the nanocomposite materials described herein can have optical,chemical, thermal, and mechanical properties engineered to suit deviceor application requirements. For certain embodiments, these variousdesirable attributes result in large part from the inherent flexibilityin engineering the surface properties of quantum dots, substantiallyindependently of their optical and electronic properties. In addition,these attributes can also result from use of organic or inorganicpolymers with tailored optical, thermal, chemical, and mechanicalproperties suitable for different devices and applications.

Alternative Applications for Engineered Nanocomposite Materials

While the application of engineered nanocomposite materials as anonlinear material is described herein, it should be recognized thatsuch materials will also find applications in a variety of areas suchas, though not limited to, engineered resonant nonlinear nanocompositematerials, engineered linear nanocomposite materials, engineeredabsorptive nanocomposite materials, engineered electro-opticnanocomposite materials, engineered thermo-optic nanocompositematerials, engineered thermal nanocomposite materials, engineered gainnanocomposite materials, engineered magneto-optic nanocompositematerials, engineered magnetic nanocomposite materials, engineeredelectronic nanocomposite materials, engineered biological nanocompositematerials, engineered optoelectronic nanocomposite materials, andengineered mechanical nanocomposite materials. It should be recognizedthat the tunable physical, chemical, electronic, and optical propertiesof the materials described herein, as well as the methods ofincorporation thereof, can be used to create materials with specificcharacteristics tailored to many other applications.

At this point, one of ordinary skill in the art will recognize variousadvantages associated with some embodiments of the invention.Embodiments of this invention provide a method of synthesizingnanocrystalline materials. Embodiments of the invention also provide amethod of synthesis that produces quantum dots with organicallyfunctionalized surfaces and a method of synthesis that produces quantumdots with surfaces passivated with oxide. Embodiments of the inventionprovide a method of synthesis that is safe, energy efficient, scalable,and cost-effective. Embodiments of this invention also provide a methodof synthesis that employs environmentally benign starting materials orcommercially available or readily prepared materials. Advantageously,embodiments of this invention provide a method of synthesis that resultsin high yield and a method of synthesis that results in very soluble andprocessable products. Also, embodiments of the invention provide amethod of synthesis that results in highly crystalline material and thatyields quantum dots with a narrow size distribution, such as with adispersion of the size distribution is less than 15% rms. Embodiments ofthis invention provide a method of synthesis that yields quantum dotswith narrow shape distribution and a method of synthesis that producesquantum dots that are uniform in composition. In addition, embodimentsof the invention provide a method of synthesis that produces quantumdots that are uniform in surface chemistry.

Each of the patent applications, patents, publications, and otherpublished documents mentioned or referred to in this specification isherein incorporated by reference in its entirety, to the same extent asif each individual patent application, patent, publication, and otherpublished document was specifically and individually indicated to beincorporated by reference.

While the present invention has been described with reference to thespecific embodiments thereof, it should be understood by those skilledin the art that various changes may be made and equivalents may besubstituted without departing from the true spirit and scope of theinvention as defined by the appended claims. In addition, manymodifications may be made to adapt a particular situation, material,composition of matter, method, process step or steps, to the objective,spirit and scope of the present invention. All such modifications areintended to be within the scope of the claims appended hereto. Inparticular, while the methods disclosed herein have been described withreference to particular steps performed in a particular order, it willbe understood that these steps may be combined, sub-divided, orre-ordered to form an equivalent method without departing from theteachings of the present invention. Accordingly, unless specificallyindicated herein, the order and grouping of the steps is not alimitation of the present invention.

1-12 (Canceled)
 13. A quantum dot comprising: a) a core including asemiconductor material Y selected from the group consisting of Si andGe; b) a shell surrounding said core; and c) at least one surface ligandcoupled to said shell, said quantum dot being substantially defect freesuch that said quantum dot exhibits photoluminescence with a quantumefficiency that is greater than 10 percent.
 14. The quantum dot of claim13, wherein a plurality of surface ligands are coupled to said shell toprovide a surface coverage between approximately 50 percent and 100percent. 15-20 (Canceled)
 21. A quantum dot comprising: a) a coreincluding a semiconductor material Y selected from the group consistingof Si and Ge; b) a ligand layer surrounding said core, said ligand layerincluding a plurality of surface ligands; and c) a shell positionedbetween said core and said ligand layer and surrounding said core, saidshell including an oxide YO_(n) with n being between approximately 0 and2, said quantum dot exhibiting photoluminescence with a quantumefficiency that is greater than 10 percent.
 22. The quantum dot of claim13, wherein a plurality of surface ligands are coupled to said shell toprovide a surface coverage between approximately 80 percent and 100percent.
 23. The quantum dot of claim 13, wherein said at least onesurface ligand contains an atom selected from the group consisting of C,O, S, Si, N, P, and Se, and said shell forms a covalent bond with saidatom.
 24. The quantum dot of claim 13, wherein said at least one surfaceligand is selected from the group consisting of alkyls, alkenyls,alkynyls, aromatics, aromatic heterocycles, conjugated aromatics,polyenes, cyanides, hydroxys, alkoxys, carboxylates, phenoxys, siloxys,cyanates, thioalkyls, thioaryls, thiocyanates, silylthios, substitutedsilyl groups, amino groups, mono-substituted amines, di-substitutedamines, imino groups, and silylaminos.
 25. The quantum dot of claim 13,wherein Y is Si, and said core has a diameter between approximately 1 nmand 20 nm.
 26. The quantum dot of claim 13, wherein Y is Ge, and saidcore has a diameter between approximately 1 nm and 50 nm.
 27. Thequantum dot of claim 13, wherein said core has a diameter betweenapproximately 1 nm and 10 nm.
 28. The quantum dot of claim 13, whereinsaid core is substantially spherical with an aspect ratio betweenapproximately 0.8 and 1.2.
 29. The quantum dot of claim 13, wherein saidshell has a thickness between approximately 0.1 nm and 5 nm.
 30. Thequantum dot of claim 13, wherein said shell includes an oxide.
 31. Thequantum dot of claim 13, wherein said shell includes an oxide YO_(n)with n being between approximately 0 and
 2. 32. The quantum dot of claim31, wherein n is between approximately 1.8 and
 2. 33. The quantum dot ofclaim 13, wherein said shell completely surrounds said core.
 34. Thequantum dot of claim 13, wherein said quantum dot exhibitsphotoluminescence with a quantum efficiency that is at least 20 percent.35. The quantum dot of claim 13, wherein said quantum dot exhibitsphotoluminescence with a quantum efficiency that is at least 50 percent.36. The quantum dot of claim 21, wherein said plurality of surfaceligands are coupled to said shell to provide a surface coverage betweenapproximately 80 percent and 100 percent.
 37. The quantum dot of claim21, wherein a surface ligand of said plurality of surface ligandscontains an atom selected from the group consisting of C, O, S, Si, N,P, and Se, and said shell forms a covalent bond with said atom.
 38. Thequantum dot of claim 21, wherein a surface ligand of said plurality ofsurface ligands is selected from the group consisting of alkyls,alkenyls, alkynyls, aromatics, aromatic heterocycles, conjugatedaromatics, polyenes, cyanides, hydroxys, alkoxys, carboxylates,phenoxys, siloxys, cyanates, thioalkyls, thioaryls, thiocyanates,silylthios, substituted silyl groups, amino groups, mono-substitutedamines, di-substituted amines, imino groups, and silylaminos.
 39. Thequantum dot of claim 21, wherein Y is Si, and said core has a diameterbetween approximately 1 nm and 20 nm.
 40. The quantum dot of claim 21,wherein Y is Ge, and said core has a diameter between approximately 1 nmand 50 nm.
 41. The quantum dot of claim 21, wherein said core has adiameter between approximately 1 nm and 10 nm.
 42. The quantum dot ofclaim 21, wherein said core is substantially spherical with an aspectratio between approximately 0.8 and 1.2.
 43. The quantum dot of claim21, wherein said shell has a thickness between approximately 0.1 nm and5 nm.
 44. The quantum dot of claim 21, wherein n is betweenapproximately 1.8 and
 2. 45. The quantum dot of claim 21, wherein saidquantum dot exhibits photoluminescence with a quantum efficiency that isat least 20 percent.
 46. The quantum dot of claim 21, wherein saidquantum dot exhibits photoluminescence with a quantum efficiency that isat least 50 percent.